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Sir Keir Starmer has alerted his cabinet to the potential for a US military strike on Iran, leading to discussions among ministers about collaborating with American forces should Tehran pose a threat to UK assets in the Middle East.

Merely a day after stating that Donald Trump had shown no signals of intending to engage in the Israel-Iran conflict, the prime minister and his cabinet assessed Britain’s potential response if the situation were to significantly escalate.

UK officials described the circumstances as “serious and unpredictable,” while Starmer’s team considered the possibility of the president targeting Iran’s nuclear sites from a joint US-UK air base located at Diego Garcia in the Indian Ocean.

The possible use of this base for US air strikes on Iran by B2 stealth bombers would draw Britain closer to the conflict. One UK official said there was not a clear “yes or no” answer to whether Washington would have to seek approval from Britain to use the base to launch an attack.

Britain has so far stayed out of the Israel-Iran war and is determined not to do anything that could lead to the closure of its embassy in Tehran, a key western diplomatic post in the Middle East.

But Starmer on Wednesday discussed with ministers and military chiefs how Britain might respond if Tehran “directly or indirectly threatened UK assets in the region”, said one person briefed on the talks.

At a meeting of a Whitehall emergency committee known as Cobra, they also reviewed the possibility of a US attack on Iran, according to officials briefed on the talks. Others who participated included intelligence bosses and US ambassador Lord Peter Mandelson.

A Downing Street spokeswoman said after the committee meeting: “Ministers were updated on efforts to support British nationals in the region and protect regional security, as well as ongoing diplomatic efforts.”

A spokesperson for Lord Richard Hermer, attorney-general, declined to comment on reports in The Spectator that he had warned that Britain’s participation in a US attack on Iran could be illegal and that UK forces could only be involved in defensive action.

But one person briefed on the matter said: “If our assets are threatened, then we have a right to self-defence, with or without the Americans. But obviously, in that case, we would want to work with the Americans given their superior power against the Iranian threat.”

While Starmer has repeatedly emphasised Israel’s right to defend itself and has said Iran cannot be allowed to develop a nuclear weapon, he has stopped short of saying he would support US involvement in the Israel-Iran conflict. 

He has continued to call for “de-escalation” of the Israel-Iran conflict — the official reason why Britain has not offered any support for the Jewish state in defending itself from air attacks directed by Tehran.

At a G7 summit in Canada on Tuesday, Starmer said there was “nothing” Trump had stated “that suggests he’s about to get involved in this conflict”.

But British officials subsequently admitted that Trump was unlikely to share his true thinking over dinner with western leaders — and that the White House’s approach to the crisis was an “iterative process”.

Israel’s ambassador in London on Tuesday said defensive support from the UK had not been discussed or requested.

This contrasts with how UK Royal Air Force fighter jets helped shoot down drones fired by Iran at Israel in April last year. RAF aircraft also provided assistance during a missile attack by Tehran on Israel last October.

The UK also helped the US last year in military strikes against Iran-backed Houthi rebels in Yemen.

Any US attack on Iran could potentially focus initially on the heavily reinforced Fordow nuclear site, which Israel lacks the bombs to destroy.

US “bunker busting” bombs capable of penetrating Fordow must be launched from American B2 stealth bombers, and Washington positioned at least six B2-As at Diego Garcia in March as it increased pressure on Tehran to agree a nuclear deal. 

The UK last month signed a £3.4bn deal to hand sovereignty of the Chagos Islands to Mauritius, while retaining a 99-year lease on the air base at Diego Garcia, the largest of the islands.

Experts said the US could launch B2 attacks from a home base in Missouri, but the additional distance and refuelling needs would be an added complication to any mission.

A UK government spokesperson said they would “not comment on hypothetical operations”.

Meanwhile, family members of British embassy and consulate staff in Tel Aviv and Jerusalem “have been temporarily withdrawn as a precautionary measure”, the Foreign, Commonwealth and Development Office said.

That decision stands in contrast to the FCDO’s current advice to UK nationals in Israel, who have been invited to register themselves with the British embassy or consulate but have not been told to leave the country.

British officials said the number of people registering for advice and other consular help was “in the low thousands”, many of them dual UK-Israeli nationals.

The FCDO has issued advice that it is still possible to leave using commercial land routes through Egypt or Jordan, but has not recommended that UK nationals try to exit Israel.

“Our key message to British nationals is to follow the advice of local authorities and stay close to shelter and to register their presence with the FCDO,” a Number 10 spokesperson said.