UK joining US-led Israeli war on Iran would be illegal, says Attorney General

The UK’s participation in a potential US-led attack on Iran on behalf of Israel could be unlawful, according to legal advice issued to Prime Minister Keir Starmer by Attorney General Lord Hermer. The warning, reported in The Telegraph, sharply limits Britain’s ability to support military action and presents a political headache for Starmer, who is under pressure to back Washington while avoiding another illegal war reminiscent of Iraq. 

Hermer, who was recently appointed as Attorney General and is a close ally of Starmer, has issued legal advice stating that any UK military involvement must be strictly limited to defensive actions, namely protecting allies, rather than participating in direct offensive operations against Iran. One official who reviewed the legal opinion remarked: “The AG has concerns about the UK playing any role in this except for defending our allies.”

This caution comes as speculation grows that US President Donald Trump may order strikes on Iranian nuclear infrastructure, using American stealth bombers and joint US-UK bases such as Diego Garcia in the Indian Ocean. While the UK retains sovereign control over the base, any US offensive would require British authorisation. 

The legal advice presents a major political dilemma for Starmer. A committed Atlanticist, the UK prime minister has signalled strong support for US-Israeli interests, but any attempt to bypass legal scrutiny in backing a unilateral military campaign may undermine his government’s legitimacy at home and abroad. Comparisons are already being drawn to Tony Blair’s controversial decision to join the US invasion of Iraq in 2003, a war widely deemed illegal by international legal experts and condemned for bypassing the UN Security Council.

Israel’s unprovoked bombing campaign inside Iran, has killed over 300 civilians. Israel’s own justification—that its strikes are defensive—is not accepted under international law unless there is an imminent threat. Any UK participation in such operations could therefore violate its obligations under the Geneva Conventions and customary international law.

READ: Iran says ready to end attacks if Israel stops assault

Meanwhile, the British government appears to be stepping back from immediate escalation. “We want to de-escalate rather than escalate,” a No. 10 spokesperson said. Foreign Secretary David Lammy has flown to Washington for urgent talks, while Defence Secretary John Healey is said to be reviewing contingency plans for RAF involvement.

Starmer’s government has authorised the deployment of six additional Typhoon fighter jets to Cyprus, with preparations underway to expand UK capacity in the Gulf. However, sources say no final decision has been made regarding the potential use of Diego Garcia.

The warnings come as Trump threatens what he described as a “very big” response to Iran’s missile attacks on Israel. Iran, in turn, has vowed to resist any foreign aggression, rejecting calls for surrender.

Legal experts say the Starmer government must avoid repeating the mistakes of the Iraq war. Any military action outside the bounds of self-defence or without UN Security Council approval is illegal.  

READ: US air defences intercept Iran’s latest missile wave after Israel’s failed attempt