Iran has been warned to expect “very serious consequences” if it breaks the terms of its nuclear deal.
Tehran has announced it will raise its enrichment of uranium beyond a limit set in its 2015 agreement with world powers.
Foreign Secretary Jeremy Hunt said he still wanted to find a way to make the deal work but was “very concerned” by the announcement from Iran.
US President Donald Trump unilaterally withdrew the US from the deal in 2018, putting its future in doubt.
The UK, France, Germany, Russia and China remain signed up to the Joint Comprehensive Plan of Action (JCPOA) with Iran, which is aimed at preventing Tehran acquiring nuclear weapons.
Mr Hunt said: “We continue to think, alongside the French and the Germans, that the most important priority is for Iran to remain nuclear free.
“We think it would be far more dangerous for the Middle East if Iran acquired nuclear weapons.
“We would still like to find a way to make this deal work, we are very concerned about the news that has come out today and we will wait for independent verification by the relevant international body before deciding what next steps (to take).
“But obviously if Iran is breaching this deal there will be very serious consequences.”
The European Union said parties to the deal are discussing a possible emergency meeting after Iran’s announcement.
Germany said it was “extremely concerned” at the announcement and called on Iran to “stop and reverse all activities inconsistent with its commitments” under the JCPOA.
Iranian officials said the new level of uranium enrichment would be reached on Sunday, but did not give further details.
Under the nuclear deal, the cap for enrichment was set at 3.67%, a percentage closely monitored by inspectors from the International Atomic Energy Agency.
Iran could diverge further from the terms of the deal within weeks, ministers warned.
Abbas Araghchi, a deputy foreign minister, said: “We will give another 60-day period, and then we will resume the reduction of our commitments.”
Enriched uranium at the 3.67% level is far below weapons-grade levels of 90%.
The decision to ramp up uranium enrichment came less than a week after Iran acknowledged breaking the deal’s 300 kilogram limit on its low-enriched uranium stockpile.
A combination of higher enrichment and a growing stockpile could put Iran closer to having the material needed for a nuclear weapon.
The breach of the JCPOA comes against a backdrop of increased tensions in the Gulf in recent weeks following an attack on two oil tankers which the UK and US have blamed on Iran and the downing of a US drone.