Iran has threatened Donald Trump with "uncontrolled consequences" if the US launches strikes.
In a letter to the United Nations, Iran has called for the UN Security Council to use its authority to ensure the US "immediately ceases" threats to Iran.
It goes on to warn if the US does strike Iran, the country would respond "decisively and proportionately".
It adds that all bases, facilities, and assets of the "hostile force" in the Middle East would be "legitimate targets".
The letter, penned by Amir-Saeid Iravani, Iran's ambassador to the United Nations, concludes: "The United States would bear full and direct responsibility for any unpredictable and uncontrolled consequences."
However, Sir Keir Starmer has blocked the US from using an RAF base to strike Iran.
It is believed Mr Trump's U-turn on the Chagos deal was in response to Downing Street's refusal to allow the US to launch bombing runs on Iran from RAF Fairford in Gloucestershire.
A US strike on Iran would then deploy from the joint US-UK military base on Diego Garcia.
Amir-Saeid Iravani raged at the US in the letter to the UN's Security Council
|GETTY
The UK does not need to consent for the US to use Diego Garcia, but must consent to allow the US to use RAF bases.
On Wednesday, Mr Trump said: "Should Iran decide not to make a deal, it may be necessary for the United States to use Diego Garcia, and the Airfield located in Fairford, in order to eradicate a potential attack by a highly unstable and dangerous regime."
RAF Fairford hosts US military personnel, and Vice President JD Vance visited the base last year.
Republican Senator Lindsey Graham accused Britain of being on "the wrong side of history" in response to the reports.
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Donald Trump once again warned Iran during his inaugural Board of Peace meeting
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He described the decision as "beyond surprising".
He added: "I’ve been a military lawyer most of my adult life. What they’re saying about the status of Diego Garcia, the joint US-UK military base, is a huge question.
"The bottom line is the largest state sponsor of terrorism on the planet is the weakest it’s been because the people of Iran have risen up by the millions to end their oppression and the United States and Israel have delivered crushing blows to the regime’s military infrastructure."
"To my friends in Britain, sitting this one out puts you on the wrong side of history and is yet another example of how much our alliances throughout Europe have degraded," the South Carolinian senator concluded.
JD Vance visited RAF Fairford last year | GETTYThe reason the UK has refused to give consent is due to concerns raised by Government lawyers the strikes would be against international law, as first reported by The Times.
Before America's strikes on Iran last year, Attorney General Lord Hermer reportedly said British involvement would be illegal.
Mr Trump has said "really bad things" would happen if an agreement with Tehran over its nuclear programme was not met.
Addressing his newly-formed "Board of Peace" on Wednesday, he said it would soon be revealed whether the US would "take it a step further" to Iran in the next 10 days.
Donald Trump has threatened Iran with 'really bad things' if it fails to sign a nuclear deal
| REUTERSAsked to clarify later by reporters, he said: "I would think that would be enough time, 10, 15 days, pretty much maximum."
He declined to be specific in terms of what a further step would look like, other than again warning it would be "really bad things".
Mr Trump has flipped his position on the Chagos Islands deal several times, describing it last month as an act of "great stupidity".
He then said it was the best deal Sir Keir could have done.
However, earlier this week, Mr Trump returned to his earlier position and said the Prime Minister was "making a big mistake".
He said: "We will always be ready, willing, and able to fight for the UK, but they have to remain strong in the face of wokeism, and other problems put before them. DO NOT GIVE AWAY DIEGO GARCIA!"