Donald Trump has called on Britain to leave the European Union without a deal if Brussels refuses to meet its demands, as he urged the Government to send Nigel Farage into the negotiations.
In his second extraordinary intervention into British politics ahead of this week's state visit, the US president suggested the UK should "walk away" from talks and refuse to pay the £39 billion divorce bill if its requests are not met.
He told the Sunday Times it was a "mistake" not to involve Brexit Party leader Mr Farage in negotiations, saying he has a "lot to offer" and is someone he likes "a lot".
Mr Trump added: "He is a very smart person. They won't bring him in. Think how well they would do if they did. They just haven't figured that out yet."
The president, who will arrive in London on Monday, said the British Government has to "get the deal closed".
He suggested: "If they don't get what they want, I would walk away... If you don't get the deal you want, if you don't get a fair deal, then you walk away."
Mr Trump added that if he was in charge, he would not pay the EU divorce bill, and he claimed it is not too late to "sue" the EU to give Britain greater "ammunition" in the talks.
He told the paper: "If I were them I wouldn't pay 50 billion dollars. That is me. I would not pay, that is a tremendous number."
Meanwhile Mr Trump vowed to "go all out" to secure a free trade deal between the UK and US within months of Britain leaving the EU.
Defying diplomatic norms for the second time in as many days after telling The Sun he backs Boris Johnson to become the next prime minister, Mr Trump also said he would have "to know" Jeremy Corbyn before authorising the sharing of highly sensitive US intelligence.
He also urged the Labour leader to "get along with the United States" if he wants Britain to continue to benefit from US military and intelligence support, the paper reported.
His intervention comes as a dozen MPs battle it out to replace Theresa May as Conservative leader, with several candidates pledging they would be prepared to take the UK out of the EU without a deal.
Meanwhile, London Mayor Sadiq Khan compared the language used by Mr Trump to that of the "fascists of the 20th century".
Writing in the Observer, Mr Khan said: "President Donald Trump is just one of the most egregious examples of a growing global threat.
"The far-right is on the rise around the world, threatening our hard-won rights and freedoms and the values that have defined our liberal, democratic societies for more than 70 years.
"Viktor Orban in Hungary, Matteo Salvini in Italy, Marine Le Pen in France and Nigel Farage here in the UK are using the same divisive tropes of the fascists of the 20th century to garner support, but with new, sinister methods to deliver their message.
"And they are gaining ground and winning power and influence in places that would have been unthinkable just a few years ago."
The first day of Mr Trump's state visit to the UK will see him have a private lunch with the Queen, tea at Clarence House with the Prince of Wales and the Duchess of Cornwall, and attend a state banquet at Buckingham Palace.