Sir Ed Davey calls on Keir Starmer to publicly back Mayor of London after Trump's verbal attack

Mr Trump hit out at Sir Sadiq Khan and accused him of only ever been able to win elections in London because of increasingly large amounts of immigration.

However on Tuesday, Downing Street denied it was failing to stand up for the London Mayor after declining to criticise the president's verbal attack.

Speaking to LBC's Tonight with Andrew Marr, Liberal Democrat leader Sir Ed said British democracy is "not to be played with by the American President."

Read more: Trump’s attack on a ‘decaying Europe’ isn’t just wrong, it’s a lecture Britain shouldn’t sit through, writes Andrew Marr

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When asked if Starmer should defend the Mayor, Sir Ed said: "I think he should.

"You'd be surprised to know that I don't agree with the Mayor of London on lots of things, but the idea that another politician from another country is interfering directly into our politics is unacceptable.

"The Prime Minister should say that publicly, whether it's defending labour politicians or saying our politics, British democracy, is not to be played with by the American President."

Andrew also asked what he thought of the UK's approach in appearing to "be very nervous, very cautious, rather timid, self censoring in how we respond to Donald Trump."

Sir Ed said: "We now have an American President who's outwardly hostile to the UK and Europe in what he says and what he's promising to do.

"The document that they released from the State Department with the White House's foreign policy stamp on it. It's staggering, and it suggests that they want to interfere into our electoral politics, in supporting parties of the hard right, wherever they are across Europe.

"That is unacceptable, that crosses more than one line, and it's vital the Prime Minister pushes back on it. We would support him in that. The Liberal Democrats have been doing that for quite a while."

Mr Trump hit out at the London Mayor during a wide-ranging interview on the state of Europe, immigration and the war in Ukraine.

In response, the official X account for the Mayor of London posted an image listing the areas where London was ranked number one in the world, including for nightlife and top travel destination.

The tweet was simply captioned: "Once more for the haters..."

When quizzed by Andrew on "the substance of [Trump's] attack" and the issues around the "scale and speed of immigration into Europe in general and Britain in particular," Sir Ed said: "I think the so called Boris Wave was unexpected and unplanned for.

"I think what happened under the Conservatives is they lost control, partly because they were so worried about the implications of their own policy of Brexit.

"We've seen a lot of analysis in the media [about] what happened in that Boris Johnson government, when they were seeing lots of Europeans leaving the UK out of jobs, out of the public sector, out of the NHS in response to Brexit.

"They were worried that that would undermine our economy, and so they were way too expansive on immigration than they were telling the British public, and they were admitting to.

"And that, I think, has caused some tensions. I've spoken about it before with the Health and Care Visas that they granted like confetti without any proper scrutiny, and I think we've not had those workers turn up at our health system. They're doing other things. So, the Conservatives cause a huge problem.

"But that doesn't mean that the analysis of Donald Trump is correct. It's complete nonsense.

"The truth is, immigration, over a number of years, has been very beneficial to the United States of America and to Europe, and I think that we don't celebrate the contribution to our economy and society enough."

Discussing a separate matter, Sir Ed also told Andrew that "a new UK-EU Customs Union would go a long, long way" to putting right the economic damage caused by Brexit.

He added he would "not see a problem' with the UK having the renegotiate post Brexit trade deals as a result, arguing if "we lost all those trade deals, they're really not very big."