The Royal Navy will join a US-led taskforce to protect merchant ships travelling in the Gulf.
The move comes amid growing tensions between the two western powers and Iran over the shipping route in the Strait of Hormuz.
Iranian forces seized British-flagged vessel Stena Impero last month, while the US has tightened sanctions on Iran.
Foreign Secretary Dominic Raab said the new maritime taskforce would give “reassurance for shipping”.
But it goes against plans laid out by his predecessor, Jeremy Hunt, for a European-led mission in the area.
The UK government confirmed last month that it would provide a Royal Navy escort, from warships HMS Duncan and HMS Montrose, for British-flagged ships passing through the strait.
The Ministry of Defence said the new mission would involve the same warships.
The US has also committed two warships to the mission, as well as aerial surveillance.
Washington has re-imposed – and latterly tightened – its own sanctions on Iran, after withdrawing from a 2015 deal to limit the country’s nuclear activities.
The UK and other European countries remain committed to the plan, but diplomatic tensions have been strained in recent months – increasing after the seizure of the Stena Impero.
Mr Raab said the UK’s decision to join the US-led mission did not change its commitment to the nuclear deal and that the government was working to “de-escalate the situation” in the Gulf.
British attempts to create a European-led coalition to provide maritime security in the Gulf – and to deter Iran – have clearly not born fruit, at least yet.
Hence the UK decision to “join an international maritime security mission”, which will be led by the US.
So far, the UK is the only other member.
The apparent change of heart has been made easier by the recent changes in Downing Street.
The UK government still says it hopes the taskforce can transition to a European-led mission – though so far no other European countries have said they’re willing to commit warships to protect merchant shipping as part of an international maritime force. Read more