The UK has expanded permission for US forces to operate from British bases to strike Iranian missile sites linked to attacks around the Strait of Hormuz.
Ministers said the change is intended to protect merchant shipping in the vital waterway and is being carried out under the principle of collective self-defence.
Downing Street stressed Britain will not take a direct combat role in any US strikes.
Until now, UK facilities were cleared only for defensive measures aimed at stopping missiles that threatened British personnel or assets.
The new agreement allows US operations to degrade missile capabilities used to target ships navigating the strait.
The bases identified for use include RAF Fairford in Gloucestershire and the British Indian Ocean Territory base at Diego Garcia.
UK military planners are also working alongside US Central Command on options to keep tankers moving through the region.
The move drew swift criticism from opposition and smaller parties.
Conservative leader Kemi Badenoch called it a dramatic reversal of earlier policy, while the Liberal Democrats and Greens urged Parliament be given a vote and warned the decision risks escalation.
Iran’s foreign ministry labelled the step as participation in aggression and warned Britain was endangering its citizens.
The UK foreign secretary described Iran’s strikes as reckless and urged an immediate halt to attacks on civilian infrastructure, including energy targets.
US President Donald Trump said the UK should have acted more quickly and blamed allies for not doing enough to reopen the shipping lane, which he called a straightforward military task.
The White House view has heightened pressure on partners to take stronger action.
Shipping through the strait has been severely disrupted.
Analysis shows fewer than 100 vessels passed the route since early March, a sharp drop from pre-conflict daily averages, at a waterway that normally handles about one-fifth of global oil shipments.
Downing Street said ministers want urgent de-escalation and a swift end to the wider conflict, while maintaining the UK’s stated aim of protecting regional commerce without becoming a direct combatant.
