Iran's threat to resume its nuclear programme today is "unnecessary and damaging", the Foreign Office has said.
Officials said that if Iran carried out its threat to resume uranium-ore conversion activities in Isfahan, further negotiations with the EU would be made "very difficult".
The department, which is leading EU attempts to reach a compromise with Iran, is pressing Tehran to wait a week before taking any action.
Yesterday Ali Aghamohammadi, a spokesman for the Iranian supreme national security council, said nuclear activities would resume if negotiations with the EU ended without acceptance of the country's legal right to be a nuclear programme.
Iran claims Europe's big three (E3) - Britain, Germany and France - agreed to set August 1st as the deadline for handing over a package of economic and political incentives in return for Tehran suspending uranium enrichment and nuclear fuel reprocessing.
But the Foreign Office has said the E3 foreign ministers and the EU high representative have written to Iran's chief nuclear negotiator confirming "full and detailed proposals would be given to Iran in a week's time" as outlined in a meeting in Geneva in May.
"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," a statement said.
It warned Tehran could be referred to the International Atomic Energy Authority (IAEA) if it presses ahead with its nuclear programme.