Britain is urging Iran not to take any steps that could jeopardize talks over Tehran's nuclear program.
The British Foreign Office Sunday said it would be "very difficult" to continue the talks if Iran carries out its threat to restart work at its Isfahan Uranium Conversion Facility.
Earlier, Iran said work at the plant will resume unless the European Union meets a Monday deadline for submitting a formal offer in the talks.
Britain, France and Germany plan to offer a package of economic and political incentives for Iran to permanently suspend its uranium-enrichment activities. Iran agreed to a temporary suspension last November.
The British Foreign Office said Sunday that Iran will receive full and detailed proposals within the next week. The United States has accused Iran of seeking nuclear weapons. Tehran says its nuclear program has strictly peaceful intentions.