On the eve of his meeting with Russia’s Vladimir Putin, US President Donald Trump has rattled allies once more by labelling the European Union a “foe”.

In an interview with CBS News’ Face the Nation program, aired on Sunday, Mr Trump lumped the EU with China and Russia as US economic adversaries.

“I think the European Union is a foe, what they do to us in trade,” he said.

Donald Tusk, president of the European Council, replied on Twitter using one of Mr Trump’s favourite stock phrases.

“America and the EU are best friends,” Mr Tusk wrote. “Whoever says we are foes is spreading fake news.”

Mr Trump’s explosive admission about the EU also comes after the British Prime Minister Theresa May revealed that the controversial US president offered her a very blunt piece of advice in regards to Brexit on his visit: Sue the EU, don’t negotiate.

A bemused Mrs May turned him down but the exchange was the latest example of the awkward dance between the US and Britain, with the two leaders attempting to put on a public show of friendliness despite clear strains over trade, the EU and their approaches to diplomacy.

Mr Trump told reporters on Friday that he had given Mrs May advice about how to deal with the EU that she found too “brutal”.

Asked in a BBC interview on Sunday what that was, Mrs May responded with an amused expression.

“He told me I should sue the EU. Not go into negotiation, sue them.” With a laugh, she added: “Actually, no. We’re going into negotiations with them.”

The meeting with Mr Putin in Helsinki will round off a whirlwind Europe tour for Mr Trump, which has seen the president ruffle feathers in Brussels at the NATO summit and be welcomed by mass protests in London and Scotland.

Mr Trump made his animosity toward the EU clear even before he left Washington, aggressively lambasting his European NATO allies and accusing them of freeloading off the US when it comes to defence spending.

When he touched down in Brussels for the NATO summit — his first stop on his European tour — Mr Trump’s first port of call was to blast Germany’s ties to Russia, alleging that a natural gas pipeline venture with Moscow has left Angela Merkel’s government “totally controlled” and “captive” to the Kremlin.

He also questioned the necessity of the intergovernmental military alliance between Europe and North America, which has formed a barricade against Soviet aggression, tweeting after a day of contentious meetings: “What good is NATO if Germany is paying Russia billions of dollars for gas and energy?”

And when the president departed Brussels for his first official visit to Britain, it was quickly steered wildly off course by a series of humiliating remarks he has made about Mrs May’s leadership — especially her handling of the tense Brexit negotiations.

In an explosive interview with The Sun newspaper published on Thursday — just as Mrs May was hosting Mr Trump at a lavish black-tie dinner — Mr Trump said the British leader’s approach likely “killed” chances of a free-trade deal with the United States.

He said he had told Mrs May how to conduct Brexit negotiations, “but she didn’t listen to me”.

He also praised Mrs May’s rival, Boris Johnson, who quit last week as foreign secretary to protest the prime minister’s Brexit plans. Mr Trump claimed Johnson would make a “great prime minister.”

The comments shocked many in Britain — even Mrs May’s opponents.

The US president later apologised and sought to soften the blow, telling reporters at a joint news conference on Friday that Mrs May is an “incredible woman” who is “doing a fantastic job” as prime minister.

Asked to rate US-UK relations, Mr Trump called them the “highest level of special”.

He added it was up to Mrs May how to handle Brexit, as long as the US “can trade and we don’t have any restrictions” on commerce with the United Kingdom.

On Sunday, Mrs May seemed to point to Mr Trump’s inconsistent advice when she said that as well as telling her to “sue” the EU, he also suggested not walking away from the negotiations.

Mrs May didn’t elaborate, and it wasn’t clear what grounds Britain would have to sue the EU, how it would work or to what purpose.