Getty ImagesDowning Street has said the sovereignty of the Falkland Islands "rests with the UK", following a report the US could review its position on the British claim to the territory.
The internal Pentagon email, which was reported by Reuters, suggested the US was considering options to punish Nato allies it believed failed to support its war on Iran.
The options discussed also included seeking to suspend Spain from Nato over its opposition to the war.
Asked about the report, a No 10 spokesman said: "The Falkland Islands have hugely voted overwhelmingly in favour of remaining a UK overseas territory, and we've always stood behind the islanders' right to self-determination and the fact that sovereignty rests with the UK."
The prime minister's official spokesman also said the government "could not be clearer about the UK's position", and that "sovereignty rests with the UK and the islanders' right to self-determination is paramount".
He continued: "We've expressed this position previously clearly and consistently to successive US administrations and nothing is going to change that."
BBC News has contacted the Pentagon about the reported email and has not been able to review the document.
The Falkland Islands review was one of a number of measures suggested in the email.
Asked about the suggestion the US could push for Spain's expulsion from Nato, an official from the military alliance said the organisation's founding treaty "does not foresee any provision for suspension of Nato membership, or expulsion".
Earlier, Spain's Prime Minister Pedro Sanchez said: "We do not work based on emails. We work with official documents and official positions taken, in this case, by the government of the United States."
The Falkland Islands, a British overseas territory in the south-west Atlantic Ocean, remain the subject of a sovereignty dispute between Britain and Argentina.
A 10-week conflict in 1982 was triggered when Argentina's military dictator, Leopoldo Galtieri, ordered his country's forces to invade the islands.
Then Prime Minister Margaret Thatcher's government sent a naval task force to recapture them.
Argentine forces surrendered, but the country still claims sovereignty over the Falklands, which it calls the Malvinas and which lie about 300 miles east of Argentina.
In the course of the conflict, 649 Argentine military personnel and 255 British military personnel lost their lives, as well as three Falkland Islanders.
While the White House is yet to comment on the report, it could prove to be another point of friction between the US and UK at a time of diplomatic tension.
The report emerged three days before King Charles and Queen Camilla were due to travel to the US, where they are due to meet US President Donald Trump at the White House.
Trump has previously said he is "not happy" with the level of support offered by the UK during its war in Iran, while Sir Keir Starmer has repeatedly said Britain will not be drawn into a wider conflict.
In April 2024, Argentina's President Javier Milei said he would set out a "roadmap" for the islands to become part of Argentina, adding that it would only be achieved via diplomatic means.
Speaking to the BBC a month later, Milei said he accepted the Falkland Islands are currently "in the hands of the UK", and that there was "no instant solution" to changing their status.
The right-wing leader also said it would take decades for the dispute to be resolved, and critcised Argentine politicians who "beat their chests demanding sovereignty of the islands, but without any result".
