US President Donald Trump has promised outgoing British Prime Minister Theresa May a very substantial and fair trade deal with the UK after its exit from the European Union (EU), asking her to “stick around” and finish the job.

"I think we�ll have a very very substantial trade deal, it�ll be a very fair deal, and I think it�s something we both want to do," Trump told May and business leaders at the start of a roundtable meeting in London on Tuesday. "We�re going to get it done."

"We are your largest partner... I think there�s a great opportunity to greatly enlarge that, especially now," Trump told the business leaders attending the breakfast meeting.

He thanked May for the job she had done as premiere and asked her to remain on the job until the deal was completed.

"I don�t know exactly what your timing is, but stick around. Let�s do this deal," Trump told May, who is due to resign on Friday after failing to deliver Brexit since taking the job in the aftermath of the June 2016 EU referendum.

She will remain in post while a contest is held within the ruling Conservative Party to replace her.

Does Trump really want May to remain PM?

It was not clear whether Trump�s comments were honest or were meant as a not-so-subtle jab at the outgoing PM given that he has already endorsed Boris Johnson -- May’s former Foreign Secretary who quit last year and became of fierce government critic -- to replace her.

“I think Boris would do a very good job. I think he would be excellent,” Trump said in an interview with the Sun newspaper on Saturday.

He told the Sun that other Conservative lawmakers had sought his endorsement except for Michael Gove, a main rival of Johnson in Tory leadership race who has criticized Trump for his policy on Iran.

In similar comments last week, Trump hailed Johnson and another UK eurosceptic politician Nigel Farage as "interesting" people. He said the two "friends" had done a good job in British politics over the past years and he was looking forward to meet them.

Trump slams May�s Brexit deal

United States President Donald Trump has slammed British Prime Minster Theresa May�s Brexit deal as favoring the EU.

Another reason Trump�s sincerity in heaping praise on May�s work sounded more like a compliment is his view of the withdrawal agreement that May snatched with the EU last years and failed three times to pass it in the House of Commons.

"Sounds like a great deal for the EU,” said Trump speaking to reporters outside the White House in November 2018.

“Because, you know, right now if you look at the deal, they may not be able to trade with us and that wouldn’t be a good thing," added the president.

May and Trump were scheduled to discuss a range of issues from trade to climate in Downing Street later on Tuesday before holding a joing press conference. Large crowds were expected to stage protests against the US President nearby.

Police had announced that it would close the road directly outside Downing Street to protect the president and his family.