Foreign Secretary Jeremy Hunt is launching a fresh bid to stop the Iran nuclear deal unravelling and ease tensions in the Gulf.
Mr Hunt will meet EU foreign ministers in Brussels to raise concerns about Iran breaching some of its commitments.
The deal, which involves Iran limiting nuclear activities in return for the easing of economic sanctions, struggled after the US withdrew in 2018.
It comes amid heightened tensions after the UK seized an Iranian oil tanker.
On Saturday, Mr Hunt said the tanker, detained by Royal Marines earlier this month, could be released if the UK is guaranteed the oil it was carrying is not bound for Syria.
At a meeting of fellow foreign ministers on Monday, Mr Hunt will work with the European partners of the deal - France and Germany - to encourage Iran to stick to its pledges.
In a joint statement issued ahead of the meeting, Britain, France and Germany reiterated their support for the deal.
But said they were "deeply troubled" by recent events in the Gulf and "concerned" over US-Iran relations.
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