Jeremy Corbyn says Donald Trump’s support of Boris Johnson is an “entirely unacceptable interference” in British politics.
The US president has described Mr Johnson, who is taking part in the Conservative leadership contest, as a “very good guy” who would be an “excellent” prime minister.
Mr Trump, who may meet the former foreign secretary during his state visit to the UK next week, also said Mr Johnson was “very talented”.
Those comments drew criticism from the Labour leader, who said the next prime minister “should be chosen not by the US president”.
Jeremy Corbyn said in a statement: “President Trump’s attempt to decide who will be Britain’s next prime minister is an entirely unacceptable interference in our country’s democracy.
“The next prime minister should be chosen not by the US president, nor by 100,000 unrepresentative Conservative Party members, but by the British people in a general election.”
Mr Trump told The Sun he had studied the battle to succeed Theresa May “very hard”.
He added: “I know the different players. But I think Boris would do a very good job. I think he would be excellent. I like him. I have always liked him.”
Mr Trump has previously described Mr Johnson as “a friend of mine” and suggested to reporters last week that he could set up a meeting with him and Brexit Party leader Nigel Farage when he arrives in the UK.
Speaking to Sky News, Mr Farage said he would be “very happy” to meet Mr Trump during the visit, but suggested no formal arrangements had yet been made.
But Mr Farage did not share the US president’s positive outlook on the prospect of Mr Johnson being prime minister, saying he had some “quite serious concerns”.
He told Sky News: “Twice [Mr Johnson] voted against Mrs May’s new European treaty – the worst deal in history – and on the third occasion he voted for it. I’ve got some quite serious concerns.”
President Trump will be welcomed by outgoing Tory leader Mrs May at the start of his three-day state visit on Monday, and suggested he could officially endorse one of the 12 candidates to replace her.
“I don’t want to say who, but other people have asked me for endorsements, yes,” he told The Sun.
“I could help anybody if I endorse them.”
The US president could well offer further commentary on British politics during his state visit next week, which will see him meet with the Queen and other senior members of the Royal Family.
He will not be meeting new mother the Duchess of Sussex – not that Mr Trump would have been anticipating a welcome reception.
Asked about Meghan’s backing of rival Hillary Clinton in the 2016 presidential election, and suggesting she would leave the US if Mr Trump won, he told The Sun: “I didn’t know that. What can I say? I didn’t know that she was nasty.”
But he added that it was “nice” to have an American in the Royal Family.