Tearful Allegra Stratton tonight 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 offering a 'profound apology' for giving the impression that she was 'making 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.
The move came hours after Boris Johnson tried to quell mounting fury by ordering an investigation by the Cabinet Secretary.
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.
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 Omicron 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 spokesman for COP26 president Alok Sharma 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.
A tearful Allegra Stratton announced she had quit offering a 'profound apology' for giving the impression that she was 'making light' of Covid rules
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.
Mr Johnson opened a raucous PMQs by saying sorry for the footage of his former press secretary and other staff joking about the gathering during lockdown
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, more mask-wearing as early as today.
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.
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 on the most pressing issue facing these islands'.
Mr Blackford said: 'Trust and leadership is a matter of life and death. Downing Street wilfully broke the rules and mocked the sacrifices we have all made, shattering the public trust. The Prime Minister is responsible for losing the trust of the people. He can no longer lead on the most pressing issue facing these islands.
'The Prime Minister has a duty, the only right and moral choice left to him: it is for his resignation. When can we expect it?'
But Mr Johnson replied: 'The party opposite and indeed the other party opposite are going to continue to play politics. I am going to get on with the job.'
Scottish Tory leader Mr Ross told STV News Mr Johnson had 'serious questions to answer on this issue'.
He said he trusted the PM, but added: 'When he made statements last week they seem to be very different to what he is saying this week and the video evidence that has emerged.
'That is a really serious issue for him, for his Government and for everyone who was involved in whatever happened in Downing Street last December.
'The public deserve answers because the public have sacrificed so much during this pandemic and they view this extremely dimly. They are understandably angry about what has happened and the fact that this has been in some ways hidden since last year.'
Mr Ross told BBC Scotland: 'If the prime minister knew about this party last December, knew about this party last week, and was still denying it, then that is the most serious allegation.
'There is absolutely no way you can mislead parliament and think you could get off with that.'
New plans for Britons to work from home and for offices to be closed are being drawn up by the Government to curb a surge in Omicron variant case numbers over the festive period,
Mr Johnson is on the verge of pushing the button on 'Plan B' measures, with a meeting of the Covid O Cabinet sub-committee due this afternoon.
Some backbenchers had suggested that Mr Johnson's could be forced to resign unless he 'holds his hands up' because of the video, which calls into question his insistence that all the rules had been followed over the No10 Christmas bash.
Former minister Tracey Crouch, MP for Chatham and Aylesford, demanded an apology.
'I am fuming! My constituents have every right to be angry,' she told Kent Online.
'Their memories of lost loved ones are traumatised knowing that they died alone, first and last Christmases passed by, and many spent what is usually a special day by themselves.
'I am not even going to begin to justify or defend a party in Downing Street. We all deserve a fulsome explanation and apology and swiftly.'
Conservative Sir Roger Gale said today that the situation bore 'all the hallmarks of another 'Barnard Castle' moment' - a reference to the Prime Minister's former aide driving 260 miles during strict lockdown conditions last year.
He said: 'This is like something out of The Thick Of It. If you wrote it, people wouldn't believe it. Boris Johnson has always been his own man and done his own thing in his own way. I think he has a certain amount of explaining to do. This is no laughing matter. I want PM to come to the dispatch box and say there either was a party or wasn't. There has to be an explanation by lunchtime today'.
Even Ant and Dec got in on the act last night, mocking Boris Johnson over the Downing Street Christmas party on I'm a Celebrity 2021 and saying: 'Evening Prime Minister… for now.'
Police are yet to launch a criminal investigation - but will be reviewing the footage before a decision is taken. It is understood that a senior officer looking at the matter will now consider the contents of the video in the coming days.
'We are aware of footage obtained by ITV News relating to alleged breaches of the Health Protection Regulations at a Government building in December 2020', a Met spokesman said. 'It is our policy not to routinely investigate retrospective breaches of the Covid-19 regulations, however the footage will form part of our considerations'.
