Donald Trump has said Jeremy Corbyn would be "so bad" for the UK while hailing Boris Johnson as "the exact right guy for the times".
The US President also said if Brexit Party leader Nigel Farage teamed up with Mr Johnson, they would be an "unstoppable force".
In a radio interview with Mr Farage on LBC, Mr Trump also criticised the Prime Minister's Brexit deal, claiming it hinders trade with the US, and said he had discussed with Mr Johnson the new agreement brokered with Brussels.
Giving his views ahead of the election, Mr Trump said: "Corbyn would be so bad for your country, he'd be so bad, he'd take you on such a bad way. He'd take you into such bad places.
"Your country has tremendous potential. It's a great country."
Talking about Mr Johnson, he said: "When you are the President of the United States you have great relationships with many of the leaders, including Boris, he's a fantastic man, and I think he's the exact right guy for the times.
"And I know that you and him will end up doing something that could be terrific if you and he get together as, you know, an unstoppable force."
Meanwhile, Mr Johnson has vowed to deliver Brexit by January at the "absolute latest" if the Tories win the election, as Mr Corbyn said the Prime Minister alone was to blame for his failure to meet his Halloween deadline.
On the day Britain was supposed to leave the EU, the two main party leaders clashed over who was responsible for the continuing deadlock over Brexit more than three years after the referendum vote.
Mr Johnson - who pledged the UK would be out by the end of October "do or die" - was determined to point the finger of blame at a Parliament dominated by Remain-supporting MPs.
"There are just too many people who are basically opposed to Brexit, who want to frustrate it," he told reporters during a visit to Addenbrooke's Hospital in Cambridge.
"It was the mandate of the people. They voted by a pretty substantial majority to do this and Parliament has simply stood in their way."
He said he had an "oven-ready" deal with Brussels which meant the UK would be out of the EU within a matter of weeks if the Conservatives were returned to power in the election on December 12, which was officially confirmed after the early Parliamentary general election Bill became law after receiving Royal Assent.
"If you vote for us and we get our programme through - which we will because it's oven-ready, it's there to go - then we can be out at the absolute latest by January next year," he said.
However, Mr Corbyn said Mr Johnson had to accept responsibility for his failure to honour the commitment he made during the Tory leadership campaign.
"He said he would rather be dead in a ditch than delay beyond today. But he has failed. And that failure is his alone." The Prime Minister moved to call the election after MPs refused to fast-track his deal with Brussels through the Commons in time for it to be ratified by the end of the month.
Mr Corbyn, kicking off Labour's election campaign with a speech in Battersea, south London, said the only way to resolve Brexit was through a second referendum.
"We need to take it out of the hands of the politicians and trust the people to have the final say," he said.
"Labour will get Brexit sorted within six months. We'll let the people decide whether to leave with a sensible deal or remain. That really isn't complicated."
Mr Corbyn moved to sharpen the battle lines between the two main parties, highlighting Labour's "ambitious and radical" programme to transform the nation.
The Labour leader sought to cast the contest as a battle between the "elites" allied with the leader of the "born to rule" and the people.
To cheers from supporters, he hit out at "tax dodgers, bad bosses, big polluters, and billionaire-owned media holding our country back", and there were chants of "not for sale" as he pledged to protect the NHS from a post-Brexit trade deal between the Conservatives and US President Donald Trump.