“I formally declare that Iran has joined the club of nuclear countries,” President Ahmadinejad told supporters in the holy city of Mashhad. The crowd broke into chants of Allahu akbar (God is greatest). Some stood and thrust their fists in the air. He added: “Enemies can’t dissuade the Iranian nation from the path of progress that it has chosen.”

The declaration raised fears that international pressure could no longer dissuade Iran from developing nuclear weapons. Experts said that its programme might now have reached the point of no return.

The White House condemned the news as evidence that Iran was “moving in the wrong direction”. A diplomat in Vienna, where the International Atomic Energy Agency (IAEA) is based, described the news as a “fork in the road”.

Western diplomats believe that if Iran chose to defy international pressure and moved to industrial-scale enrichment it could have enough material for a weapon in three years.

Iran’s move was a direct rebuff to the UN Security Council, which last month set the country a 30-day deadline for persuading the world that its research on nuclear power did not conceal a weapons programme. Mohammed ElBaradei, director-general of the IAEA, arrives in Tehran tomorrow to try to persuade Iran to freeze its programme.

The Tehran regime has rejected demands to halt the programme, saying it had a right to develop nuclear energy for peaceful purposes.

John Bolton, the hawkish US ambassador to the UN, said the announcement “shows that they are not paying any attention to what the Security Council has said because they are clearly continuing with their enrichment activities”. A Foreign Office spokesman said that the remarks were “not helpful”.

Weekend reports in Washington claimed that the US was preparing for military strikes against the Tehran regime. President Bush called the reports wild speculation but has maintained that the military option remains.

China, however, has reiterated its support for a diplomatic solution. Early today its UN ambassador, who is President of the Security Council, said that military action or sanctions would be counter-productive. “We still believe that negotiations and a diplomatic solution are the best way out of it,” Wang Guangya said.

Uranium enrichment is the greatest technical obstacle to a country making nuclear weapons — or even fuel for reactors, which Iran says is its sole aim. It requires precision-crafted centrifuges made out of hardened steel, which must spin very fast without crashing. Once uranium has been enriched to the level needed by a reactor, it takes about as long again to enrich it to weapons grade.

Hojatoleslam Ali Akbar Hashemi Rafsanjani, the former Iranian president who now heads Tehran’s expediency council, said yesterday that Iran had begun enriching uranium in 164 centrifuges.

This is no more than a pilot plant. Iran has stated it that hopes to run 50,000 centrifuges at its nuclear plant in Natanz. This year it announced plans to install the first 3,000.

Nuclear experts said that there was no proof Iran had mastered the technology. David Albright, a former UN weapons inspector, said that he was “a little sceptical that Iran can run the cascade continuously”, without machines crashing.

But Gary Samore, a specialist in Iran’s nuclear programme at the MacArthur Foundation in Chicago, said that once they had mastered the work, the Iranians would have passed the point when the outside world could prevent them getting the technology.

Ray Takeyh, of the Washington-based Council on Foreign Relations, said that Tehran was “upping the ante” and wanted to fracture the international coalition. It has had some success, with Germany, Russia and China recently calling for fresh talks.

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