Downing Street has been forced to insist that Britain will not yield sovereignty over the Falkland Islands, after a leaked Pentagon email proposed the US should reassess its support for the UK’s claim on the islands because of a lack of British support over Iran.

The memo reflected ways in which the Trump administration could punish Britain for failing to follow the US lead in bombing Iran, and comes before a potentially fraught three-day state visit to the US by King Charles.

It argued, according to Reuters, that the US could review a policy of endorsing European claims to longstanding “imperial possessions”, and highlighted the Falklands, subject of the 1982 war between Britain and Argentina.

The report immediately prompted pushback from the UK government, opposition leaders, veterans and the Falklands, underscoring a rapid decline in the tone of Anglo-American relations in the past few weeks.

“We could not be clearer about the UK’s position on the Falkland Islands,” the prime minister’s spokesperson said when asked about the email. “It’s longstanding. It’s unchanged. Sovereignty rests with the UK, and the islands’ right to self-determination is paramount.”

The US reiterated its neutrality over the Falkland Islands on Friday, according to a US state department spokesperson.

“Our position on The Islands remains one of neutrality. We acknowledge that there are conflicting claims of sovereignty between Argentina and the UK,” the spokesperson said, adding that the US recognizes “de facto United Kingdom administration” of the archipelago without taking sides on sovereignty claims.

Though it was vague and there was no immediate sign of it being adopted, the US-leaked Falklands proposal appeared deliberately designed to provoke a reaction in the UK, where memories of the war linger.

Britain recaptured the Falklands in June 1982 at the end of a bitter 74-day conflict, in which 255 members of the British armed forces, 649 Argentinians and three Falkland Islanders were killed.

The US gave discreet support to the UK, providing satellite and signals intelligence and Stinger handheld missiles, in a war that began with an Argentinian invasion of the islands and that it was not immediately certain Britain would win.

Though there has been no return to hostilities since, the status of the islands, which are 8,000 miles from the UK and 300 miles from Argentina, remains disputed by Argentina, now led by Trump’s ally President Javier Milei.

Argentine foreign minister Pablo Quirno responded to the reports by reiterating his country’s willingness to resume bilateral negotiations for a “peaceful and definitive solution”.

Quirno characterised the current status of the islands, which are sometimes called the Malvinas, as a “colonial situation” and expressed gratitude for international support regarding Argentina’s claims.

“By history, by right, and by conviction: the Malvinas are Argentine,” Quirno said in a post on X.

Kemi Badenoch, the Conservative leader, said the Falklands were “British territory” while Nigel Farage, who leads Reform UK, said he would tell Milei, in a meeting scheduled for later this year, that the status of the islands was “non-negotiable”.

Ed Davey, the Lib Dem leader, repeated that the king’s state visit to the US should be cancelled at the last minute. “This unreliable, damaging president cannot keep insulting our country,” he said.

The king and Queen Camilla are due to fly into Washington on Monday. The day after, Charles will make a rare address to both houses of Congress, before the couple visit the White House for a state dinner hosted by Trump in an acutely sensitive transatlantic moment.

This week, peers said the UK-US relationship was “under greater strain today than at any point since the second world war”. George Robertson, the chair of the international relations and defence committee, said that Britain’s high military dependence on the US was “no longer tenable”.

Downing Street said on Friday it was not worried about a possible change of heart by Trump. “We have got one of the most important security and defence relationships, if not the closest, that the world has ever seen, and it continues,” Starmer’s spokesperson said.

The Falkland Islands’ government added that it had “complete confidence” in the UK’s commitment to uphold its right to self-determination and cited the result of a 2013 referendum where “99.8% of voters, on a turnout of 92%” voted in favour of remaining a British overseas territory.

Simon Weston, a Falklands veteran who suffered severe burns during the fighting, said that islanders had been able to live peacefully for 44 years since the war. “Just because he [Trump] feels slighted because people didn’t rush to his war that he created, that he started, he did not need to start this,” he told Times Radio.

The leaked memo appears to have been drawn up in response to White House frustration that the UK and other members of Nato did not provide sufficient support for the US-led 38-day bombing campaign against Iran.

It argued that Spain should be suspended from Nato for refusing to allow US warplanes to be based in or fly over the country during Operation Epic Fury, though it is not clear if there are mechanisms for doing so.

Though the exact wording of the email has not been made public, US officials spoke as its contents were as summarised. Kingsley Wilson, the Pentagon press secretary, said: “As President Trump has said, despite everything that the United States has done for our Nato allies, they were not there for us.

“The war department [Department of Defense] will ensure that the president has credible options to ensure that our allies are no longer a paper tiger and instead do their part. We have no further comment on any internal deliberations to that effect.”

Starmer largely kept the UK out of the Iran war, but unlike other European countries did allow the US to fly B-1 and B-52 bombers from British bases on what were deemed defensive missions, including against Iranian missile launchers and anything used to target shipping in the strait of Hormuz.

Trump, however, repeatedly complained about the lack of military support provided by the UK, complaining that Britain only wanted to help in protecting the strait after the war was over, that the Royal Navy’s aircraft carriers were “toys”, and compared Starmer to Neville Chamberlain.

The public outbursts are markedly different from Ronald Reagan’s behaviour in 1982, when the US president made clear his support for the British prime minister, Margaret Thatcher, in private. “We will do what we can to assist you. Sincerely, Ron,” he wrote shortly after the initial Argentinian invasion.

Spain’s prime minister, Pedro Sánchez, said that Spain was a “loyal” Nato member. “The Spanish government’s position is clear: absolute cooperation with allies, but always within the framework of international law,” the Spanish leader said.

Sánchez – who has been the most vociferous European critic of the US and Israel’s war in Iran – had angered Trump by refusing the US permission to use jointly operated bases in southern Spain.

On Friday, he renewed his criticisms. “The crisis that this illegal war has brought to the Middle East shows the failure of brute force – and has prompted demands for international law to be respected and for the multilateral order to be safeguarded and reinforced,” he said.