Britain and Europe have warned Donald Trump that any US attack on Iran risks endangering Westerners being held by the Islamic Republic.

Diplomatic sources told The Telegraph that Washington had also been told that military intervention could trigger a wave of terror attacks by sleeper cells across Europe.

It comes as part of an effort by Britain, France and Germany to broker a diplomatic end to Israel’s war with Iran.

Mr Trump has approved plans for a US strike on Iran, but has not yet given a final order.

European foreign ministers will meet their Iranian counterpart for talks in Geneva, Switzerland, on Friday.

David Lammy, Jean-Noel Barrot and Johann Wadephul will be joined by Kaja Kallas, the EU’s top diplomat, for what could emerge as showdown negotiations to avoid an all-out war in the Middle East.

As the conflict between Israel and Iran continued to escalate on Thursday, Sir Keir Starmer called for “cool heads and a return to diplomacy”. The Prime Minister’s spokesman added: “We would not want to see anything that ramps up the situation.”

Emmanuel Macron, the French president, issued a similar plea, calling for an urgent end to Israel’s war to bring Mr Trump back from the brink of becoming involved.

Mr Macron also told aides that he believed Israel’s bombing campaign had moved beyond its original targets of Iran’s nuclear production and ballistic missile facilities.

The talks in Geneva on Friday are aimed at kick-starting a diplomatic process in coordination with Washington and Tehran before any further escalations.

A source told The Telegraph that the UK, France, Germany and EU had all urged restraint from Washington because of a possible threat to detainees held by the Islamic regime.

Two British nationals, a man and a woman, were arrested in Iran in February and accused of security-related offences. In recent years, at least 15 people with links to the UK have been arrested in the country and accused of espionage offences.

In April, the EU hit Iran with sanctions over what it called a policy of state-sponsored hostage taking.

It was also relayed that a vulnerable Iran could lash out by using its network of proxies to conduct a terror campaign on the streets of Europe. This warning was designed to resonate with Mr Trump, who himself was a target of an Iranian murder-for-hire plot.

Tehran has been behind similar assassination attempts in Britain, where an Iranian television journalist was stabbed in London by two men acting on behalf of the regime.

Last year, Ken McCallum, the director of the MI5 intelligence service, warned that at least 20 Iranian-backed plots had been foiled by the British authorities since 2022.

Mr Lammy, the Foreign Secretary, was expected to meet Marco Rubio, the US Secretary of State and national security adviser, in Washington to deliver these messages.

The American earlier this week also held talks with Ms Kallas, who afterwards said: “I urge all parties to exercise restraint and prevent further escalation.”