"We could not be clearer about the UK's position on the Falkland Islands. It is long standing, it is unchanged," a spokesperson for Prime Minister Keir Starmer told reporters on Friday.
"Sovereignty rests with the UK and the islands' right to self-determination is paramount. It's been our consistent position and will remain the case," the spokesperson said, adding that Britain had expressed that position "clearly and consistently to successive US administrations".
The Pentagon email outlined options for the United States to punish NATO allies it believes failed to support US operations in the US-Israeli war on Iran, including suspending Spain from the alliance and considering reassessing US diplomatic support for longstanding European "imperial possessions", such as the Falkland Islands near Argentina.
Britain and Argentina fought a brief war in 1982 over the islands after Argentina made a failed bid to take them.
Some 650 Argentine and 255 British service personnel died before Argentina surrendered.
Asked if Starmer thought this was an attempt by the US to put pressure on him to join the Iran war, his spokesperson said: "He has spoken about that and he has also spoken about how that pressure does not affect him, and he will always act in the national interest, and that will always remain the case."
President Donald Trump has harshly criticised NATO allies for not sending their navies to help open the Strait of Hormuz, which was closed to global shipping following the start of the air war on February 28.
He has also declared he is considering withdrawing from the alliance.
Britain, France and others say that joining the US naval blockade would amount to entering the war, but that they would be willing to help keep the strait open once there was a lasting ceasefire or the conflict ended.
But Trump administration officials have stressed that NATO cannot be a one-way street.
They have expressed frustration with Spain, which has also irked the US administration with its refusal to hike defence spending to five per cent of GDP, adamant that it can meet its obligations with less.
"We do not work off emails. We work off official documents and government positions, in this case of the United States," Spanish Prime Minister Sanchez said when asked about the report before a meeting of European Union leaders in Cyprus to discuss topics including NATO's mutual assistance clause, adding that Spain was a "loyal partner" to NATO.
An option in the email envisions suspending "difficult" countries such as Spain from important or prestigious positions at NATO.
Asked whether it was possible to suspend a NATO ally, a NATO official said "NATO's Founding Treaty does not foresee any provision for suspension of NATO membership".
