Warning on Iran 'nukes'.
The regime suspended plans to build a bomb last year due to international pressure.
The US now believes it is resuming its programme, though Iran insists it is for civilian use.
The Foreign Office said in a statement: "We have received reports that the Iranians have decided to restart their uranium conversion facility at Isfahan.
"This would be an unnecessary and damaging step by Iran. We are seeking clarification of Iran's intentions. We urge them not to take any unilateral step."
European states have threatened to refer Iran to the UN Security Council if the country resumes its nuclear activities.
Economic incentives for Iran to stop are also being considered.