Sir Keir Starmer has blocked Donald Trump from using RAF bases to strike Iran.
The Government is yet to give consent for the US to use military bases in the UK to launch bombing runs, because of concerns they would breach international law.
The US is putting together contingency plans for a direct attack on Iran, with long-range bombing and refuelling aircraft sent to the Middle East in recent days. It is the largest US military build-up in the region since 2003, when the country went to war in Iraq.
Any US strike on Iran would likely use the Diego Garcia military base in the Chagos Islands – the British overseas territory that the UK is planning to give away to Mauritius – and European air bases in allied countries.
The US does not need consent to use Diego Garcia, but must ask the UK government before using RAF bases.
The UK’s reluctance to allow the US to use bases in this country is thought to have led Mr Trump, the US president, to withdraw his support for Sir Keir’s Chagos deal, under which Britain will give away sovereignty over the archipelago, including the Diego Garcia military base, which is shared with the US.
On Wednesday, Mr Trump told the UK to “remain strong in the face of wokeism” and not to “give away” the islands. Posting on Truth Social, the US president 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.”
The request was made amid rising tension between the US and Iran over Tehran’s refusal to sign a deal to curb its nuclear programme.
Mr Trump’s comments are understood to be a reference to a US request for permission to launch long-range bombing runs from RAF Fairford, a military base in Gloucestershire that hosts US Air Force personnel.
However, the UK has yet to give consent owing to concerns raised by government lawyers, first reported by The Times, that participating in the strikes could be a breach of international law.
Government sources said the UK is unlikely to support any pre-emptive military strike on Iran, following a similar refusal to participate in the US president’s attack on Iran’s nuclear facilities last summer.
The UK could be held responsible for an illegal attack on Iran by the US under a UN edict passed in 2001, if it had “knowledge of the circumstances of the internationally wrongful act”.
After Mr Trump’s strikes on Iran last summer, ministers repeatedly refused to say whether Britain’s lawyers believed the strikes were legal.
In his message on Wednesday, Mr Trump laid out the bones of a legal justification for bombing Iran, arguing that a strike could “eradicate a potential attack by a highly unstable and dangerous Regime… [that] would potentially be made on the United Kingdom, as well as other friendly Countries”.
The regime in Tehran has said it will respond to any attack Credit: Majid Asgaripour/Reuters
The ultimate responsibility for government legal advice on military strikes lies with Lord Hermer, the Attorney General, although it is unclear whether he has been directly consulted about the US’s use of RAF bases.
Before last year’s US strikes on Iran, it was reported that Lord Hermer had produced legal advice warning that any British involvement beyond defending UK assets in the region would be unlawful.
The Attorney General reportedly told ministers he had “concerns about the UK playing any role in this except for defending our allies”, including Israel.
The full legal advice was never published, but Whitehall sources told The Telegraph on Thursday that the UK was still unlikely to participate in any pre-emptive strikes on Iran, indicating that the British legal position has not changed.
Over the last month, the Ministry of Defence has deployed additional Typhoon and F-35 fighter jets to the region in anticipation of any Iranian response to an attack by the US.
US military build-up in the Middle East is at similar levels to 2003, when it went to war in Iraq, prompting Iran to engage in naval drills in the Persian Gulf
Mr Trump could also launch a strike on Iran from Diego Garcia, a base it is only required to “notify” Britain about using, under the terms of a treaty between the two countries.
Under Sir Keir’s Chagos deal, the sovereignty of the islands will pass to Mauritius, while the base will be leased back at a cost of £35bn over 99 years.
Mr Trump has repeatedly flipped between support and opposition of the deal, which he said last month was an act of “great stupidity”.
He then said that he believed the agreement was the best that Sir Keir could have struck to maintain control of the base amid legal challenges from Mauritius in the international courts.
On Wednesday, the US president returned to his earlier position, writing on Truth Social: “We will always be ready, willing, and able to fight for the U.K., but they have to remain strong in the face of Wokeism, and other problems put before them. DO NOT GIVE AWAY DIEGO GARCIA!”