Iran offered what it described as "a new formula" to resolve the dispute over its nuclear programme today and said it was ready for talks.

Details of the offer, which was presented to diplomats in Tehran today, more than a week before a UN deadline for Iran to stop enriching uranium, were not immediately available but Iranian officials said that they expected negotiations to be able to resume after months of deadlock.

"Although there is no justification for the other parties’ illegal move to refer Iran’s case to the Security Council... the answer was prepared... to pave the way for fair talks," Iran’s chief nuclear negotiator, Ali Larijani, told the country's student news agency.

"Iran is prepared to hold serious talks from August 23."

There was no word from Western diplomats, who said before the meeting that they were expecting "an ambiguous" response from Tehran.

Iran's statement was given to the ambassadors of Britain, China, Russia, France and Germany at the offices of the Supreme National Security Council in Tehran. The United States, which has no diplomatic presence in Tehran, was represented by the Swiss ambassador.

The Iranians decided to reply today more than a week before the official UN deadline of August 31, which western diplomats have said remains the formal close of this stage of negotiations.

In the UN Security Council resolution passed on July 31, Iran was offered a range of incentives in return for ceasing the enrichment of uranium, a process that readies the fuel for use in a nuclear reactor but which is also the first stage in the development of atomic weapons.

Iran insists that its fledgling nuclear programme is intended to generate electricity, but America and the "E3" group of Britain, France and Germany have voiced suspicions that Tehran wants to build a nuclear bomb.

The incentives designed to soften the Iranian stance included promises to build up trade, diplomatic and other relations with Tehran, including the first face-to-face talks with America in nearly three decades.

Iran would be allowed to build light-water reactors to produce nuclear power but the nuclear fuel would be imported, probably from Russia. In return Iran would have to cease its uranium enrichment.

In the run-up to today's statement, Iranian officials had mixed intransigence with hints that their response would allow grounds for optimism.

While Iran has repeatedly said it will never give up its right to enrich uranium, it has suggested that it might suspend enrichment to allow for further negotiations.

Yesterday, Ayatollah Ali Khamenei, the Supreme Leader of Iran, promised that Iran would continue its nuclear programme unabated. Hours earlier, inspectors from the International Atomic Energy Agency (IAEA) were refused access to an underground bunker thought to be used for uranium enrichment.

But Mohammed Saeedi, the deputy head of the Atomic Energy Organization of Iran, described the coming reply as "an exceptional opportunity" to restart negotiations.

"Iran’s response to the package is a comprehensive reply that can open the way for resumption of talks for a final agreement," he said.

Nonetheless, Mr Saeedi highlighted sources of Iranian dissatisfaction with the UN offer. The package of incentives made no mention to the section of the Nuclear Nonproliferation Treaty that gives countries the right to pursue nuclear technology for peaceful purposes, according to Mr Saeedi.

As today's talks took place in Tehran, Iranian officials arrived in Moscow to discuss the launch of the country's first nuclear power station, which has been built and supplied with fuel by Russia. The £423 million plant, in the Iranian city of Bushehr, is expected to start producing electricity at the end of the year.

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