Donald Trump has risked sparking a diplomatic row ahead of his state visit to the UK by declaring that Boris Johnson would be "an excellent" prime minister.

Wading into the Conservative leadership contest, the US president described the foreign secretary as a "very good guy" who would "do a very good" as the successor to Theresa May.

Mr Trump arrives in the UK on Monday for a three-day state visit. It is highly unconventional for a sitting US president to involve themselves in a UK political leadership contest.

His comments came as Nigel Farage, leader of the Brexit Party, said he had been “banned” from meeting the US president.

In an interview with The Sun ahead of his visit, Mr Trump said: “I've actually have studied it very hard. I know the different players. I think Boris would do a very good job. I think he would be excellent.”

He added: “I like him. I have always liked him. I don’t know that he is going to be chosen, but I think he is a very good guy, a very talented person.

“He has been very positive about me and our country.”

Who could succeed Theresa May as Conservative leader? Show all 9 1 /9 Who could succeed Theresa May as Conservative leader? Who could succeed Theresa May as Conservative leader? Boris Johnson Former foreign secretary Boris Johnson has long been hopeful, he previously stood in the leadership contest that followed the Brexit vote and has at many times since been thought to be maneuvering himself towards the goal. He remains a darling of the party's right wing, particularly those in the ERG, and is the most popular choice among Tory voters but his leadership bid would be fiercely opposed by many MPs PA Who could succeed Theresa May as Conservative leader? Michael Gove Environment secretary Michael Gove is another member who has long wanted to be leader. He has lately been known for rousing his party in the commons, his recent speeches on the Brexit deal and Labour's no confidence motion have overshadowed the Prime Minister's. He has been loyal to the Prime Minister, partly to shed his reputation as a backstabber who abandoned Boris Johnson to stand against him in the 2016 leadership election Getty Who could succeed Theresa May as Conservative leader? Dominic Raab Former Brexit secretary Dominic Raab has emerged as a favourite to be the Brexiteer candidate in a contest to succeed to Ms May. He displayed a grip on detail in his role as Brexit secretary. When asked recently if he would like to become prime minister he replied "never say never" Getty Who could succeed Theresa May as Conservative leader? Rory Stewart International development secretary Rory Stewart is pitching himself as the sensible candidate, promising to rule out both a second referendum and a no-deal Brexit. He was only recently promoted to the cabinet, previously serving as prisons minister, where he caught headlines with a pledge to resign if he could not reduce levels of violence within a year PA Who could succeed Theresa May as Conservative leader? Esther McVey The former work and pensions secretary announced that she will be standing for the leadership when May leaves. McVey is the first to explicitly state that she intends to stand. She resigned from the cabinet in protest over May's Brexit deal AFP/Getty Who could succeed Theresa May as Conservative leader? Sajid Javid Home secretary Sajid Javid is said to have a plan in place for a leadership race. He made headlines over Christmas when he declared that people smuggling over the English channel was a "major incident" and more recently when he revoked the citizenship of ISIS bride Shamima Begum. Son of a bus driver, he wants the Conservatives to be seen as the party of social mobility PA Who could succeed Theresa May as Conservative leader? Jeremy Hunt Foreign secretary Jeremy Hunt was recently thought to be the favourite in the event of a leadership race as he could sell himself as the man to unite the party. Critics worry that his long stint as health secretary could return to haunt him at a general election. He has reportedly been holding meetings with Tory MPs over breakfast to promote his leadership PA Who could succeed Theresa May as Conservative leader? Andrea Leadsom Following the Prime Minister's second defeat over her Brexit deal, Leader of the house Andrea Leadsom hosted a dinner party at which "leadership was the only topic of conversation", The Times heard. Leadsom ran against Theresa May in the 2016 leadership election before dropping out, allowing May to become Prime Minister AFP/Getty Who could succeed Theresa May as Conservative leader? Priti Patel Former international development secretary Priti Patel is thought to be positioning herself as a contender. One MP told The Independent "she knows she's from the right of the party, the part which is going to choose the next leader, so she's reminding everyone she's there." Patel left the government late in 2017 after it emerged that she had held undisclosed meetings with Israeli officials PA

Mr Trump also claimed that other candidates in the Tory leadership race had asked him to endorse them.

He said: “Other people have asked me for an endorsement too. I have been asked for endorsements”.

He added: “I could help anybody if I endorse them. I mean, we’ve had endorsement where they have gone up for 40, 50 points at a shot.

“Now that is here, but I understand over there it would be a great endorsement.”

Mr Trump has previously spoken of his admiration for Mr Johnson and Mr Farage.

According to The Times, senior White House officials are preparing for a meeting between Mr Trump and Mr Johnson.

Tory leadership race: Boris Johnson in profile

Mr Farage, however, said he has been “banned” from meeting Mr Trump during his visit.

He told the Daily Express that the US president's team had been told by No 10 not to arrange a meeting.