Boris Johnson has been accused of “dog whistle racism” and “base politics of the worse kind” after remarks about Barack Obama's Kenyan heritage – as Nigel Farage echoed the controversial comments.

The London Mayor - who is backing the campaign for Britain to leave the European Union - criticised the US President for his intervention in the EU referendum debate, adding his attitude to Britain might be based on his “part-Kenyan” heritage and “dislike of the British Empire”.

In a column for The Sun, Mr Johnson referred to the removal of a bust of Winston Churchill from the Oval office when Mr Obama became President.

"No one was sure whether the President had himself been involved in the decision," he said.

"Some said it was a snub to Britain. Some said it was a symbol of the part-Kenyan President's ancestral dislike of the British Empire - of which Churchill had been such a fervent defender."

Ukip leader Nigel Farage echoed the Mayor's comments, adding: "His first day in the White House, he had the bust of Winston Churchill removed from the Oval Office.

"Because of his grandfather and Kenya and colonialisation, I think Obama has a bit of a grudge against this country."

But cross-party politicians, diplomats and commentators attacked Mr Johnson over the remarks.

“Mask slips again. Boris part-Kenyan Obama comment is yet another example of dog whistle racism from senior Tories. He should withdraw it,” said the Shadow Chancellor John McDonnell.

Labour MP Chuka Umunna, said: "Once again we see the ugly face of the Tory party. The nasty party is back. Zac Goldsmith has played on Sadiq Khan's Muslim heritage to try to link him with radical extremists, and today Boris Johnson has played on Barack Obama's Kenyan ancestry to question his motives around the EU referendum debate.

"This is beyond the pale and base politics of the worst kind. We may have come to expect this from Donald Trump – but Goldsmith and Boris should know better, and Londoners deserve better.”

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Conservative MP Nicholas Soames, who is the grandson of the late Sir Winston Churchill, was not impressed by the comment piece. “Appalling article by Boris Johnson in [The] Sun, totally wrong on almost everything,” he said.

Richard Newby, the Liberal Democrat chief whip in the Lords, said Johnson’s remarks were despicable. “Boris Johnson suggesting that Obama is anti-British because father was Kenyan. Despicable. Desperate. Demeaning," Mr Newby said.

Two former senior British diplomats also joined those criticising Mr Johnson, over his claims in the Sun newspaper.

Sir Stephen Wall, former British permanent representative to the European Union, said: "Boris Johnson's comment implying the President of the United States is driven by his ancestral dislike of the British Empire is demeaning to the debate. Using that type of language does not reflect Britain's standing in the world or the country we aspire to be.

"As our most important ally, President Obama has the right to offer his view and he has made it clear that being in Europe magnifies British influence and enhances Britain's global leadership."

And former UK ambassador to Washington Lord Kerr said: "The US has an interest in Britain, its closest ally, being stronger, safer and better off in the EU – not weaker, out on its own. To claim that the American president has no right to say what he believes, and speak up for US political, economic and business interests is typical Boris bluff and bluster."

Labour frontbencher Diane Abbott added: "Boris dismissing President Obama as 'half-Kenyan' reflects the worst Tea Party rhetoric".

The claim over removing the bust of Britain’s wartime leader from the Oval office, however, was debunked by the Washington Post in January.

“There is no evidence that Obama personally decided to return the bust; given the economic crisis at the time, one imagines he had bigger issues on his mind,” a journalist at the Post wrote.

Asked how David Cameron viewed Mr Johnson's comments on the president's "half-Kenyan" heritage, the Prime Minister's official spokeswoman said: "It is important to engage with the facts.

"If you look at the issue the mayor of London was talking about, which related to the bust of Churchill in the Oval Office, they have been clear that this suggestion that he asked for it to be moved and that it is a failure of the president's appreciation of the special relationship is false.