U.S. President Donald Trump made an extraordinary intervention into the British election on Thursday as campaigning got underway ahead of the December 12 poll.

In a radio interview, Trump criticized Boris Johnson's deal to take the UK out of the European Union, called for the prime minister to join forces with the Brexit Party and warned opposition leader Jeremy Corbyn would be "so bad" for the country.

Trump is overwhelmingly unpopular in the UK according to polling, with YouGov finding that 67 percent of Britons hold a negative view of the U.S. president.

Brexit deal bashed

Trump, interviewed by Brexit Party leader Nigel Farage on radio station LBC, argued that under "certain aspects" of Johnson's Brexit agreement, the U.S. and UK "can't make a trade deal."

"To be honest with you, this deal, under certain aspects of the deal, you can't do it, you can't do it, you can't trade," Trump claimed.

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The comment directly contradicts Johnson, who Trump described as a "fantastic man", and could be a boost for Farage's party. The prime minister says under his agreement the UK will be able to strike free trade deals around the world.

"Under this new deal the whole of the UK will leave the EU customs union, which means we can strike our own free trade deals," a Downing Street spokesman insisted on Thursday.

Labour weaponizes Trump

The president's intervention was leaped on by the opposition Labour Party, which has put a claim that the U.S. could try to buy up parts of the National Health Service (NHS) at the center of its campaign.

"Trump is trying to interfere in UK election to get his friend Boris Johnson elected," Corbyn tweeted in response to the U.S. president's claim that the Labour leader "would be so bad for your country."

Trump denied the claim that the U.S. could attempt to play a greater role in the NHS. "I don't even know where [it] started with respect to us taking over your healthcare system. I mean it's so ridiculous," Trump said. "I think Corbyn put that out there."

The future of the NHS is a central part of any election campaign, and Johnson's Conservatives have made investment into the health service a central part of their platform. The prime minister campaigned on Thursday under the slogan "Back Boris for more NHS funding."

Johnson-Farage pact backed

Trump also said Farage, a longtime supporter of the U.S. president, should strike a deal with Johnson's Conservatives to create an "unstoppable force."

"He [Johnson] is a fantastic man and I think he's the exact right guy for the times," Trump told the Brexit Party leader. "And I know that you and him will end up doing something that could be terrific. If you and he get together it's, you know, unstoppable force."

Farage said he would be "right behind" Johnson if the prime minister dropped the deal he struck with the EU and instead went for a "clean break" Brexit.

The Brexit Party leader is a critical figure in the election campaign. If his party decides to challenge the Conservatives in key target seats, it could draw vital votes away from Johnson and undercut his chances of forming a majority government.