The international dispute over Iran's nuclear programme appears to be escalating, with Tehran threatening to resume uranium conversion.
Iran says it wants to resume nuclear enrichment
The UK Foreign Office urged Iran not to take unilateral steps that could jeopardise talks with three European Union nations - known as the E3.
The remarks came after a top Iranian official set a Sunday deadline for the EU to propose economic incentives.
The UK - the current EU president - said these would be given in a week.
This was in accordance with the decisions of the Geneva meeting in May between Iran and the three European countries - Britain, France and Germany - as well as the EU's foreign policy chief, Javier Solana, said a Foreign Office (FCO) spokesman.
This is threatening to become a dangerous escalation, says the BBC's Jon Leyne.
The US believes Iran is trying to build a nuclear bomb, but Iran insists its programme is for civilian use only.
Iran suspended all uranium conversion and enrichment activities in November 2004 as a result of international pressure.
However, it has always insisted that the suspension was temporary and that it would resume some of its nuclear activities regardless of EU proposals.
The European states have threatened to refer Iran to the UN Security Council for possible sanctions if Iran resumes its nuclear activities.
The UK reaction came after Iran said it would resume nuclear activities at the Isfahan plant on Monday if the Europeans had not submitted their proposals.
"If we do not receive the EU proposal today [Sunday], tomorrow morning we will start part of the activities in Isfahan's uranium conversion facility," Ali Aghamohammadi, spokesman for the Supreme National Security Council, told state television.
"This will be under the supervision of UN inspectors," he said.
The FCO spokesman said 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 which would contravene the Paris agreement as that would make it very difficult to continue with the E3/Iran negotiations.
"Should the Iranians persist, we will as a first step consult urgently with our partners on the board of the IAEA [International Atomic Energy Agency]."
Iran appears to be hardening its position but it is not clear if this is just a way of putting pressure on Europe before the talks or a serious threat, says the BBC's Frances Harrison in Tehran.
Earlier this week, outgoing President Mohammad Khatami said he hoped EU diplomats would allow for a resumption of enrichment activities, but that Iran would begin again in any case.
Mahmoud Ahmadinejad, the conservative former Tehran mayor who was elected Iran's president last month, has said he wants to continue the nuclear programme.
Uranium enrichment can be used to fuel nuclear power stations, but can also provide material for nuclear weapons.