The UK is preparing to provide support if Donald Trump decides to bomb Iran, despite a last-ditch diplomatic push to persuade the United States against military action.

The US President is to make a decision within the next two weeks on whether the US will join Israeli strikes, the White House said on Thursday – and suggested that Trump believes there is a prospect of a diplomatic outcome to the conflict.

Reacting to Trump’s statement a No10 source said:

“We welcome any development that offers a path to de-escalation and diplomacy, which has been our consistent position throughout.”

Trump is keeping both his allies and enemies guessing on whether he will join Israel’s aerial bombardment, potentially by providing American bunker-busting bombs to wipe out Iranian nuclear facilities buried deep in a mountainside in Fordo.

Trump has reportedly already approved an attack plan but has yet to decide whether to go ahead or not.

Sir Keir Starmer issued a direct appeal to Trump to de-escalate on Thursday.

But the Government is also planning in case Trump decides to strike Iran and is weighing up the different options that will then be open to the UK, it is understood.

This includes joining in a US bombing campaign, although this is seen as very unlikely. Britain could also decide to do nothing.

US bunker busting bombs

The most likely option, however, is helping to facilitate US air strikes, for example, if Trump requests British support for using the UK’s Diego Garcia military base in the Chagos Islands in the Indian Ocean.

The base is home to B-2 stealth bombers – the only aircraft that can carry specialised 30,000lb bunker-busting bombs that would be used to hit the Iranian underground nuclear enrichment facility at Fordo.

Mauritius will not need to be notified in advance of any strikes, despite the recent deal with the UK handing over sovereignty of the Chagos Islands, and will have no veto.

The US is expected to continue to build up their military presence in the Middle East over the coming days ahead of a possible devastating strike on Fordo.

Israeli insiders expect the US President to either order the strike or present an ultimatum to Tehran to make a deal.

Tel Aviv would agree to a diplomatic outcome as long as Iran’s nuclear programme and ballistic missile facilities are destroyed. Tehran has made clear, in response to Trump’s demands that Ayatollah Khamenei “surrender” over the conflict, that it will not “grovel” to Washington and reach an agreement.

Israel ‘does not want regime change’

But there are hopes in diplomatic circles that talks in Switzerland on Friday, involving the foreign ministers of the UK, France and the EU as well as representatives of the Iranian regime, will encourage Tehran to make a deal with the US.

Richard Pater, the director of Britain Israel Communications and Research Centre (BICOM), said that Israel does not want regime change in Iran despite the country’s defence minister warning that Ayatollah Khamenei “cannot be allowed to exist”.

Pater added: “There are two scenarios, both realistic. The US strikes the Fordo nuclear site, the one big one left, using their heavy bombers.

“Israel sees it as important but not essential to have America’s help – they can spend the next few days chipping away with smaller ordnance into the mountain. It would be an easier operation with the US.

“Clearly, the preparation is coming into place. Carriers and planes are on their way. There is a feeling that by Saturday, they will be ready operationally.

“The head of Centcom (US Central Command), Michael Kurilla, will present the feasibility [of a strike] to Trump. Trump can then say he has stopped Iran from having nukes.

“Or he can not go in and use this as leverage – saying to Iran with a knife to their throats to ‘make a deal [on Iran’s nuclear capability] and I will get the Israelis to back off’.”

Asked if Israel would accept the US doing a deal with Iran, Pater said: “If Israel completes their military operation [targeting nuclear sites and missiles] and America gets a deal [with Iran], then that would be an acceptable diplomatic outcome for Israel. And Trump can still say he was the arbiter of all of this.”

Doubts UK support would meet international law

The UK providing limited support was backed by Commons Foreign Affairs Committee chair Emily Thornberry, who told The i Paper: “The best place to be if the Americans do decide [to join the bombing of Iran], is that we’re not obstructive but we’re not helpful.

“If they want to use their bases in Diego Garcia, we don’t say no, but equally, we don’t offer assistance. Because it’s consistent with what we are saying.”

But the Labour MP, a barrister who specialised in human rights and is a former shadow attorney general, did not think strikes against Iranian nuclear sites would be legal under international law.

She said: “You can use force to defend yourself if you are attacked, you can use force to defend an ally.

“But I don’t think that’s the position here, I think it’s just stretching it too far.”

Attorney General Lord Hermer is reported to have raised legal concerns about any potential British involvement in the conflict beyond defending its allies, something which could limit the extent of any support for the US if Trump decides to act militarily.

Evacuation planning for Britons in Israel

While the UK was planning for all scenarios, Starmer continued to urge Trump not to bomb Iran by insisting a deal on the country’s nuclear plan is the best way to resolve the conflict in the Middle East.

The Prime Minister warned against “escalation” as he insisted “discussions with the US” are “the way to resolve this issue”.

A Whitehall source said the Foreign Secretary supports planned nuclear deal talks in Geneva between Germany, France, Britain and Iran on Friday and believes a diplomatic solution is needed “to address the nuclear issue for the long term”.

It suggests that Lammy could warn Rubio that destroying Iranian nuclear facilities now could make Tehran more determined to acquire a bomb in the future.

The Government is meanwhile understood to be optimistic that Israeli aerial dominance over Iran means the country’s airspace may soon reopen, allowing evacuation flights.

The UK’s military focus is on contingency planning, with 14 jets at RAF Akrotiri in Cyprus where preparations are being made for evacuating Britons from Israel, likely on commercial or charter flights.