Boris Johnson squirmed under questioning over the Downing Street Christmas party tonight - after a senior aide quit in tears for joking about the 'illegal' gathering.
Facing the nation at a press conference tonight the Prime Minister repeated his claim that no Covid laws had been breached when aides in No10 met over wine and cheese last December.
But after days of firm denials he added a new caveat, saying that the rules had not been broken 'as far as I'm aware' ahead of a probe into the affair to be led by top civil servant Simon Case.
And he vowed that if rules were found to have been broken by the probe, those responsible faced 'proper sanctions' - including possibly criminal investigation.
The stark nature of the allegations facing the Prime Minister and his top team were made stark by chief scientific adviser Sir Patrick Vallance.
Standing next to the PM tonight as Mr Johnson introduced tough new Plan B restrictions, Sir Patrick bluntly said: 'Measures only work because people follow them. It is very important everyone follows them.'
Mr Johnson told the press conference that Cabinet Secretary Mr Case's investigation will look into 'what took place on December 18' rather than other alleged parties.
'As for other events, dates ... as far as I'm aware, to the best of my knowledge we have followed the rules throughout,' he said.
'Indeed, as far as I'm aware, the rules were followed on December 18 as well.'
Earlier Allegra Stratton had dramatically resigned as a government aide in the wake of bombshell video showing her giggling about an 'illegal' Christmas party in No10.
The former spokeswoman for the PM announced she had quit her £125,000 a year role offering a 'profound apology' for appearing to 'make light' of Covid rules.
In emotional comments to journalists near her London home, Ms Stratton said: 'My remarks seemed to make light of rules, rules that people were doing everything to obey. That was never my intention.
'I will regret those remarks for the rest of my days.'
In the video, Ms Stratton suggested she had not personally attended the party - quipping that she 'went home' instead.
Boris Johnson said there was 'no excuse' for the video showing No 10 aides joking about an alleged lockdown party, but that Allegra Stratton had been a 'fine colleague'.
'Allegra Stratton has resigned and I wanted to pay tribute to her because she has been, in spite of what everybody has seen, and again, I make no excuses for the frivolity with which the subject was handed in that rehearsal that people saw in that clip,' the Prime Minister told a press briefing.
'There can be no excuse for it. I can totally understand how infuriating it was.
'But I want to say that Allegra has been a fine colleague, has achieved a great deal in her time in Government, and was a particularly effective spokesman for Cop26 - she coined the coal, cars, cash, trees agenda and helped to marshal and rally the world behind the agreement.
'If you'll forgive me, I wanted to say that because it is a sad day for her, as well as an infuriating event for many people around the county.'
The move came hours after Boris Johnson mounted a desperate effort to put a lid on the chaos threatening to derail the response to the Omicron variant. But the government seems paralysed by inability to disentangle the video scandal from the looming Covid crisis.
SAGE has warned ministers that Omicron hospital admissions could breach 1,000 a day in England by the end of the year without tougher restrictions to stem the spread of the super-mutant Covid variant.
The powerful Covid O Cabinet subcommittee has been meeting to sign off on 'Plan B' curbs, although the prospect of the proposals being finalised by tonight seems to be receding.
Mr Johnson opened a raucous PMQs by saying sorry for the footage of Ms Stratton and other staff joking about the gathering which happened on December 18 last year, during lockdown.
Facing the nation at a press conference tonight the Prime Minister repeated his claim that no Covid laws had been breached when aides in No10 met over wine and cheese last December.
Standing next to the PM tonight as Mr Johnson introduced tough new Plan B restrictions, Sir Patrick (far left) bluntly said: 'Measures only work because people follow them. It is very important everyone follows them.'
He said the government's top civil servant Simon Case - who is not believed to have attended the party - would be looking into the situation.
Mr Johnson said he was 'sickened' and 'disciplinary action' would be taken against any staff found to have breached the regulations, but insisted he had been 'repeatedly' assured the rules had been followed.
The premier appealed for the public to focus on the threat from the mutant strain - with Plan B measures expected to be confirmed as early as this afternoon. But Keir Starmer shot back that Mr Johnson had lost the 'moral authority' to impose restrictions.
Tory MPs also hammered the PM by branding the Covid switch a 'diversionary tactic' from the No10 party row, while Scottish Conservative leader Douglas Ross warned he will have to resign if he knew about the gathering when he issued denials.
Meanwhile, former chief aide Dominic Cummings has waded in by alleging that there was a party in Mr Johnson's grace-and-favour flat on November 13 last year, the day he was ousted from Downing Street.
Mr Case will also look at claimed former Education Secretary Gavin Williamson hosted a bash during the festive season, but significantly he will not consider any other events - meaning nothing the PM personally attended is in scope.
In her statement tonight, a weeping Ms Stratton said: 'The British people have made immense sacrifices in the battle against COVID-19.
'I now fear that my comments in the leaked video on 20 of December last year have now become a distraction in that fight.'
Ms Stratton, who has been the PM's spokesman for COP26 since the idea of her doing daily televised briefings was ditched, went on: 'Working in government is an immense privilege. I tried to do right by you all. To behave with civility and decency and act to the high standards you expect of No10.
'I will always be proud of what was achieved at COP26 in Glasgow and the progress made on coal, cars, cash and trees.
'This country and the PMs leadership on climate change and nature will make a lasting difference to the whole world.
'It has been an honour to play a part in that. I understand the anger and frustration that people feel.
'To all of you who lost loved ones, who endured intolerable loneliness, and who struggled with your businesses, I am truly sorry. This afternoon I am offering my resignation to the Prime Minister.'
On another tumultuous day for the government and country:
The core Covid O Cabinet sub-committee is being presented with a series of options papers for tightening restrictions against the Omicron variant; A SAGE meeting yesterday is reported to have concluded there is a danger hospital admissions could top 1,000 a day by the end of the year without action; A snap poll by Savanta ComRes has found that 54 per cent of the public think Mr Johnson should resign over the Downing Street Christmas party debacle; The top civil servant at the Department for Education has admitted she could be disciplined over attending a drinks event last Christmas.
A tearful Allegra Stratton announced she had quit offering a 'profound apology' for giving the impression that she was 'making light' of Covid rules
Ms Stratton - who quipped on the video that she had not attended the Christmas party - wept as she read her resignation statement tonight
Boris Johnson (left) appealed for the public to focus on the threat from the Omicron strain - with Plan B measures expected to be confirmed as early as this afternoon. But Keir Starmer (right) shot back that Mr Johnson had lost the 'moral authority' to impose restrictions.
Former Scottish Conservative leader Ruth Davidson said the PM's stance was not 'remotely defensible'
Former chief aide Dominic Cummings has waded in by alleging that there was a party in Mr Johnson's grace-and-favour flat on November 13 last year, the day he was ousted from Downing Street
Kicking off the clashes in the House earlier, Mr Johnson said: 'I understand and share the anger up and down the country at seeing No 10 staff seeming to make light of lockdown measures, and I can understand how infuriating it must be to think that people who have been setting the rules have not been following the rules because I was also furious to see that clip.
'I apologise unreservedly for the offence that it has caused up and down the country and I apologise for the impression that it gives.
'But I repeat that I have been repeatedly assured since these allegations emerged that there was no party and that no Covid rules were broken, and that is what I have been repeatedly assured.
'I have asked the cabinet secretary to establish all the facts and to report back as soon as possible - and it goes without saying that if those rules were broken then there will be disciplinary action for all those involved.'
Mr Johnson said No10 would hand over any relevant evidence to the police if they ask for it.
But the Labour Leader said: 'An internal investigation into what happened – the situation is as clear as day. I thought last week was bad enough.
'Surely the Prime Minister hasn't now going to start pretending that the first he knew about this was last night? Surely we have all watched the video of the Prime Minister's staff including his personal spokesperson.
'They knew there was a party, they knew it was against the rules, they knew they couldn't admit it, and they thought it was funny.
'It is obvious was happened. Ant and Dec are ahead of the Prime Minister on this. The Prime Minister has been caught red-handed. Why doesn't he end the investigation right now by just admitting it?'
Mr Johnson replied: 'Because I have been repeatedly assured that no rules were broken. I understand public anxiety about this… but there is a risk of doing a grave injustice to people who have frankly obeyed the rules.
'That is why the Cabinet Secretary will be conducting an investigation, and that is why there will be requisite disciplinary action if necessary.'
In one brutal attack, Sir Keir contrasted Mr Johnson's leadership with that of the Queen.
'Her Majesty the Queen sat alone when she marked the passing of the man she'd been married to for 73 years. Leadership, sacrifice – that's what gives leaders the moral authority to lead,' the Labour leader said.
'Does the Prime Minister think he has the moral authority to lead and to ask the British people to stick to the rules?'
Mr Johnson accused Sir Keir of trying to 'muddy the waters, to confuse the public and to cause needless confusion about the guidance' during the pandemic.
Conservative MPs have been turning on the PM, with demands for him to provide an 'explanation' and even warnings that misleading parliament on what happened will be a 'resigning matter'.
One government source told MailOnline that the situation was an 'absolute joke', adding: 'A friend said to me ''you look like a bunch of c***s''. It was hard to argue.'
Another MP said of Mr Johnson's leadership: 'I'm tired of it. He has to go. Clean sweep. It's unsustainable'. A third said: 'It confirms my suspicion of the sheer arrogance and hypocrisy of those orchestrating lockdown measures.'
Baroness Warsi, a former Cabinet minister turned critic, said 'every minister, parliamentarian and staffer' at the alleged party 'must resign now'. And former Scottish Conservative leader Ruth Davidson said his stance was not 'remotely defensible'.
Mr Johnson had hoped to grab the news agenda this week with a slew of crime measures, after weeks of torrid sleaze headlines.
But Health Secretary Sajid Javid pulled out out of interviews this morning following the emergence of the footage showing the PM's former press secretary Allegra Stratton laughing about the lockdown-busting gathering last year.
As bereaved families branded the revelations a 'bullet to the chest', Scotland Yard is set to probe the exchanges which took place during a rehearsal for a media briefing.
Attorney General Suella Braverman was seen going into the building this morning, although it is not clear whether her visit was related. Mr Javid has also entered, ignoring questions about why he ducked interviews.
Extraordinarily, there are signs that the government is preparing to announce a move to 'Plan B' Covid restrictions to combat the Omicron strain - including blanket orders to work from home and more mask-wearing.
Such a shift had not been expected before the end of this week, and Mr Johnson immediately faced accusations he is deploying the 'dead cat' tactic - making a big newsworthy announcement in order to distract attention from another crisis.
Tory MP William Wragg said in the House: 'Covid passes will not increase uptake of the vaccine but will create a segregated society.
'Is my Right Honourable Friend the PM aware that very few will be convinced by this diversionary tactic?'
Mr Johnson replied: 'No decisions will be taken without consulting the Cabinet.'
Speaker Lindsay Hoyle said he had not yet been notified that there will be a ministerial statement on Covid later. 'It must be here first,' Sir Lindsay warned.
In a move that could bounce No10 into activating its winter 'Plan B', SAGE has said it is 'highly likely' Omicron will make up the majority of British infections within 'a few weeks' and put 'unsustainable pressure' on the NHS.
Leaked minutes from SAGE's emergency meeting today showed that the group also expect the highly-evolved strain to trigger several thousand admissions per day at a peak in January if it is allowed to spread unchecked.
The 'Covid O' Cabinet committee has met for crunch talks this afternoon and Mr Johnson is expected to sign off on plans for a blanket order to work from home where possible, more mask-wearing, vaccine passports and stricter isolation rules for close contacts of infected people either today or tomorrow.
That is despite increasingly hostility on the premier's own parliamentary benches, with many MPs sceptical about a creeep back towards lockdown.
There is growing pressure on the Government to tighten restrictions after the total number of British Omicron cases rose to 568 today, with the highly evolved variant now in every country in the UK and almost every region of England.
Experts warn thousands of cases are flying under the radar because not all samples are analysed for variants and Omicron is estimated to be doubling every two or three days — much faster than when Delta exploded on the scene.
There are currently 757 daily hospital admissions across the UK and 680 in England. SAGE members had previously suggested that 1,200 daily admissions would be the trigger point for more restrictions with Delta.
Not a single one of the UK's confirmed Omicron cases has been hospitalised with the virus but it takes several weeks to fall seriously unwell and there are early indications in South Africa that it might cause milder illness than past variants.
But even if the new strain is milder, experts warn that if it can infect significantly more people than Delta, it will cause bigger surges in hospital admissions than its predecessor.
In the leaked SAGE minutes, seen by the BBC, the group said: 'With the speed of growth seen, decision makers will need to consider response measures urgently to reduce transmission if the aim is to reduce the likelihood of unsustainable pressure on the NHS.'
The scientists insisted that tightening restrictions will give the UK precious time to delay the wave and get more boosters into arms. They said they expect jabs to hold up against severe disease and death.
Earlier, Professor Neil Ferguson, a key SAGE member whose modelling prompted the initial lockdown last spring, today admitted that another full-blown shutdown could be on the cards to tackle Omicron.
He said light measures like WFH 'wouldn't stop it but it could slow it down' and extend the doubling time to five or six days. 'That doesn't seem like a lot, but it actually is potentially a lot in terms of allowing us to characterise this virus better and boost population immunity,' he told BBC Radio 4's Today programme.
BBC Breakfast took the extraordinary step of empty-chairing Mr Javid this morning as vaccines minister Maggie Throup also pulled out of her planned round of regional television interviews. On ITV's Good Morning Britain, host Susanna Reid swiped that the government was 'gaslighting' by pretending there is not any need to answer questions.
The footage of Ms Stratton was filmed on December 22 last year – four days after the alleged 'boozy' party and when London was under strict Tier 3 coronavirus curbs. The revelation follows a week of tortured denials from No 10 that there was a 'party' - even though dozens of staff allegedly exchanged 'secret Santa' gifts and drank past midnight at an event said to have included party games.
In the Commons, SNP Westminster leader Ian Blackford demanded Mr Johnson's resignation, saying he 'can no longer lead
