Washington’s ambassador to the UK has warned that it would be “a mistake” to cancel King Charles’s planned state visit to the US.

Asked about calls to postpone the historic royal trip because of the ongoing conflict in the Middle East, Warren Stephens said it would be wrong for it to be scrapped or delayed and insisted the trip should go ahead.

In a Q&A session after his speech to the British Chambers of Commerce conference in London on Thursday, Mr Stephens refused to confirm the visit, which has not been officially announced, but added: “I think he will go and I think it will be a very meaningful trip for him.”

The diplomat said US House Speaker Mike Johnson had made an offer for Charles to address both Houses of Congress while he was there.

It comes as The Independent has been told that Congressional and Senate leaders have been told to prepare for a joint meeting to hear the King speak during the week beginning 27 April.

But the visit is being overshadowed by the war in Iran and Donald Trump’s repeated criticism of Keir Starmer. The US president suggested the PM was “no Winston Churchill” after he initially refused permission for the US to use RAF bases to launch attacks on Iran.

A source in Washington involved with the organisation of the visit told The Independent that President Trump “will go ballistic” if the visit is cancelled and will “blame the prime minister”.

The president regards Charles as a personal friend, and the visit is being timed to coincide with the celebrations in the US of the 250th anniversary of the Declaration of Independence.

While the government has insisted that the state visit is a matter for Buckingham Palace, it is understood that decisions on these Royal events are taken in consultation with the Foreign Office as an extension of UK diplomacy.

Senior politicians have called for the visit to be scrapped in the wake of the Iran war, including the chair of the Foreign Affairs Select Committee, Dame Emily Thornberry.

“If it was to go ahead next month – the dates haven’t been confirmed, but everybody seems to think it’s going to be next month – it would go ahead against a backdrop of a war and that, I think, is quite difficult. And the last thing that we want to do is to have their Majesties embarrassed,” she told BBC Radio 4.

Liberal Democrat leader Sir Ed Davey has also called for the visit to be cancelled.

“At a time when Trump has launched an illegal war that is devastating the Middle East and pushing up energy bills for British families, it’s clear this visit should not go ahead,” the Lib Dem leader added.

“A state visit from our King would be seen as yet another huge diplomatic coup for President Trump, so it should not be given to someone who repeatedly insults and damages our country.”

A recent YouGov poll revealed that almost half of those surveyed (49 per cent) believe the visit should be cancelled, while a third (33 per cent) support it going ahead.

Green voters are the most opposed to the visit (70 per cent), with Lib Dems (65 per cent) and Labour voters (62 per cent) not far behind, the poll found. Only Reform UK voters are firmly in favour (63 per cent), while Tories are split 45 per cent to 43 per cent on the subject.