The Liberal Democrat leader has called on Sir Keir Starmer to cancel the King's US state visit over the war in Iran.
The King is due to visit the US next month to commemorate 250 years since the country's independence.
But there have been calls for the monarch to postpone the visit in the wake of the escalating row between Washington and London.
President Donald Trump has repeatedly lashed out at Sir Keir over the UK's refusal to allow the US to launch its initial attacks on Iran from British bases.
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Sir Ed called on the prime minister to cancel the king's trip due to the "illegal" war launched more than a week ago that has seen retaliatory airstrikes across the Middle East.
The Lib Dem leader said on Sunday: "Keir Starmer should advise the King that the state visit to the US scheduled for April should be called off.
"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.
"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."
Sir Keir's decision not to grant permission for the first wave of military action prompted a raft of criticism from Mr Trump in recent days, including a personal attack on the prime minister as being "not Winston Churchill".
Sir Keir later granted permission for "defensive" US action against Iranian missile sites from UK bases.
After reports that the UK was preparing an aircraft carrier to go to the Middle East, the US president posted on Saturday that the UK was "finally giving thought" to sending two, but "we don't need them any longer".
He added: "But we will remember. We don't need people that join Wars after we've already won!".
Downing Street confirmed this afternoon that Mr Trump has since spoken to Sir Keir in a phone call.
The call was understood to have lasted 20 minutes.
A readout from Downing Street about what was discussed said: "The leaders began by discussing the latest situation in the Middle East and the military cooperation between the UK and US through the use of RAF bases in support of the collective self-defence of partners in the region.
"The prime minister also shared his heartfelt condolences with President Trump and the American people following the deaths of six US soldiers.
"They looked forward to speaking again "
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Sir Keir has defended his decision not to permit the US to use British bases in the opening assault against the Tehran regime, suggesting it could have been unlawful.
Speaking to Sky News's Trevor Phillips earlier, Foreign Secretary Yvette Cooper said the prime minister would not turn to "rhetoric or hyperbole" and would focus on "calm, steady decision-making".
The prime minister agreed a week ago to allow the US to strike Iran defensively from RAF Fairford in Gloucestershire and Diego Garcia, an island in the Indian Ocean.
This weekend four American bombers landed at Fairford and the US started using British bases for "specific defensive operations" to prevent Iran firing missiles in the region.
(c) Sky News 2026: Lib Dem leader calls for King's US state visit to be cancelled after Trump's criticism of Starmer
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