President Donald Trump has warned he might not share vital intelligence with the UK if Jeremy Corbyn becomes Prime Minister.
The US president says he would consider not passing information between the US and UK if the Labour leader moves in to No 10 without meeting him.
In an interview with the Sunday Times Mr Trump said he needed to 'get to know' Mr Corbyn before deciding whether to share his country's most sensitive secrets.
He added the Labour leader was 'making a mistake' by failing to make America a friend.
The US and the UK's military and intelligence agencies work closely together.
'I would have to know him. I would have to meet. I don’t know him but I would certainly — before I would answer that question I would have to get to know him a little bit,' Mr Trump said.
The US president says he would consider not passing information between the US and UK if the Labour leader moves in to No 10 without meeting him
Today members of the public had to pass through security fences in Regents Park, erected around the residence of the US Ambassador to the UK, Winfield House
Concrete crash barriers and steel security fencing enclosed police at the site this afternoon
The Labour leader refused to attend tomorrow evening's state dinner - but the US president says he is 'not offended' by the decision.
It could have an impact on the Five Eyes intelligence sharing network that connects spies from America, the UK, Canada, Australia and New Zealand.
The US president is also expected to ask the British Government to step away from plans to involve technology giant Huawei in the UK's 5G expansion.
He said: 'Look, national security is so important, so we all have to be very careful together. And the UK understands that very well. Very well.'
It comes as Donald Trump urged the British Government to involve Nigel Farage in Brexit negotiations with the EU.
The President hailed the Brexit Party leader as a 'very smart person' who could have bolstered Theresa May's botched deal that she failed to push through Parliament.
On the eve of his state visit to the UK, his criticism of the Prime Minister's decision to omit Mr Farage from talks with Brussels will likely put noses out of joint in Downing Street.
Tourists take photographs as a US flag hangs above them from Horse Guards Parade in London today
In an interview with the Sunday Times Mr Trump said he needed to 'get to know' Mr Corbyn before deciding whether to share his country's most sensitive secrets
Security preparations are underway as teams prepare for the US presidents arrive tomorrow morning
High fences have been placed around the edge of Winfield House, where Donald Trump will be staying in London from tomorrow
Armed guards were on patrol at the gates of Winfield House this morning - as the president's security team ensure his safety
Donald Trump has sensationally urged the British government to involve Nigel Farage in Brexit negotiations with the EU
Mr Farage struck up a friendship with Mr Trump in 2016 when he endorsed his Presidential campaign and even spoke at one of his Mississippi rallies
'I like Nigel a lot. He has a lot to offer. He is a very smart person. They won't bring him in,' he told the Sunday Times.
'Think how well they would do if they did. They just haven't figured that out yet.'
Mr Farage struck up a friendship with Mr Trump in 2016 when he endorsed his Presidential campaign and spoke at one of his Mississippi rallies.
And Mr Trump - a former businessman who prides himself on his ability to strike deals - said that if he were tasked with leading negotiations he would 'walk away' if he could not broker the agreement he wanted.
He also suggested that if Mrs May had brought Mr Farage to the table earlier she may have been able to take a withdrawal agreement back to the Commons which MPs could swallow.
The President hailed the Brexit Party leader as a 'very smart person' who could have bolstered Theresa May's botched deal which she failed to push through Parliament
His branding of Mr Farage's exclusion as a 'mistake' by the Prime Minister comes a day after revelations he waded into the Tory leadership contest by seemingly backing Boris Johnson.
And tonight, it emerged ministers and courtiers fear President Trump could embarrass the Queen by publicly backing Brexit in front of her at a Buckingham Palace banquet tomorrow.
Full details of Trump's state visit to Britain have now been revealed, including all the royals he will meet over the three days
Officials involved in the planning of the controversial State visit are braced for Mr Trump to use his banquet address to heap praise on the UK's decision to quit the EU, despite the Royal Family's scrupulous attempts to remain above the political fray.
Palace sources last night insisted the Queen would be 'more than capable' of dealing with such controversy and Foreign Office insiders said they had not broached the subject with Washington.
A Whitehall source said: 'We've learnt that asking them to steer the President off a subject tends to have the opposite effect.'
Roads leading towards Winfield House have been cordoned off from today to ensure any security threats are lowered
A sign on the metal fencing placed around Winfield House reads: 'Do not enter. A police security operation is taking place. This includes armed patrols, dog patrols, monitored CCTV and other measures'
Aides fear that President Trump will breach protocol by endorsing Brexit in front of the Queen (pictured with the Queen in 2018)
And a Palace insider added: 'We are not in the business of telling a foreign leader what they can and cannot say for political reasons.'
However, such an intervention by Mr Trump would cause fury, given the lengths to which the Palace has gone to distance the Queen from the divisive debate about Britain's relationship with the EU.
When one newspaper accused the Queen of supporting Brexit in 2016, the Palace made a furious and unprecedented complaint to the Press regulator.
Comments about Brexit would mark a second breach of protocol by the President following his unusual endorsement of Boris Johnson to be the next Prime Minister – before Theresa May has officially resigned.
A detailed plan of what Mr Trump will do during his state visit to the UK. He is expected to meet the Queen, Theresa May, and attend a lavish state banquet at Buckingham Palace
And after his diplomatic gaffe of backing the current Tory leadership frontrunner, Mr Trump is poised to anger Downing Street further by meeting Brexit Party boss Nigel Farage.
Mr Trump last week described him as a 'friend' and it is thought the pair could be guests at a dinner hosted by the President on Tuesday evening at the official London residence of the US Ambassador. Spokesmen for both men said last night they had yet to receive such an invitation.
In a separate development, Foreign Secretary Jeremy Hunt was forced to deny that he had pleaded with Mr Trump to endorse his bid for No 10, after the President said in an interview that some of Mr Johnson's leadership rivals had sought his blessing.
A Foreign Office source said that while it was true that Mr Hunt was in regular contact with Mr Trump's son-in-law and adviser Jared Kushner, it was 'categorically untrue' that he had sought an endorsement.
Palace sources insist that the Queen will be able to cope with anything that Mr Trump might say. The president is a noted fan of Brexit Party leader Nigel Farage
Foreign Office insiders said they had not broached the subject of the president's speech with Washington
Jeremy Corbyn condemned Mr Trump's intervention in the leadership contest. 'President Trump's attempt to decide who will be Britain's next Prime Minister is an entirely unacceptable interference in our country's democracy,' said the Labour leader, who has snubbed an invite to tomorrow's State banquet.
Mr Trump responded to the Labour leader's decision by saying Mr Corbyn was 'making a mistake' in not attending because as a potential future Prime Minister 'he would want to get along with the United States'.
Commons Speaker John Bercow is also boycotting the banquet.
The Government last night pleaded with the public to welcome Mr Trump to London, arguing that 'the special relationship is generational, not one person'.
Home Office Minister Ben Wallace said: 'The relationships are daily and long-developed relationships. You don't unpick that. The special relationship is not Presidents and Prime Ministers.'
The US president also praised hard Brexiteer Boris Johnson, and it looks as if he will meet with the Foreign Secretary during his visit
