Iran has informed the International Atomic Energy Agency that it has been building a previously undeclared nuclear facility to enrich uranium, raising fears that Tehran is closer to acquiring an atomic bomb than has been predicted up until now.
The presence of a secret second site - built inside a mountain near the holy Shia city of Qum - has been known about by American and other Western intelligence agencies for some time, although nothing has been revealed until now.
Iran's formal letter to the IAEA in Vienna, sent on Monday, pre-empted an announcement to be made today by President Obama, Gordon Brown and President Sarkozy of France before the opening of the G20 economic summit in Pittsburgh, in which Tehran will be accused of building the secret facility about 100 miles southwest of the Iranian capital.
Although the CIA and National Security Agency (NSA) has been tracking construction of the plant for several years, Mr Obama decided it was time to put maximum pressure on Tehran by revealing its existence.
Reports from Washington indicate that Iran had learnt of the West's uncovering of its second plant and moved to declare it formally to the IAEA.
Iran wrote a brief, cryptic letter to the IAEA saying it now had a "pilot plant" under construction, whose existence it had not revealed. Iran's first and officially declared facility is at Natanz in southern Iran.
Marc Vidricaire, IAEA spokesman, said: "I can confirm that on September 21 Iran informed the IAEA in a letter that a new pilot fuel-enrichment plant is under construction in the country. The letter stated that the enrichment level would be up to 5 per cent."
Uranium enriched to around 5 per cent can be used as nuclear fuel, but has to be enriched to around 90 per cent to be effective in a nuclear weapon. The United States, Britain and other Western countries believe that Iran has been attempting, at its plant at Natanz, to achieve a higher enrichment of uranium, although the plant has been subject to IAEA inspections.
Iran has enough low-enriched uranium but has yet to develop the higher grade for a weapon.
The IAEA spokesman said• "Iran assured the agency in the letter that further complementary information will be provided in an appropriate and due time. In response, the IAEA has requested Iran to provide specific information and access to the facility as soon as possible. This will allow the agency to assess safeguards verification requirements for the facility."
He said the IAEA was told by Tehran that no nuclear material had been introduced into the facility.
The revelation of the second plant further complicates the chances of any meaningful progress at the scheduled talks on October 1 between Iran and six world powers, the first in more than a year. The five permanent United Nations Security Council members and Germany will be pressing Iran to scale back on its enrichment activities. But Tehran has declared that it will not bargain on enrichment.
Mr Obama and the two other leaders will today demand that Iran opens the doors of the secret site to IAEA inspectors.
Pressure has been mounting on Iran this week to halt its nuclear programme. Britain indicated at the UN that "far tougher sanctions" must await Iran if it failed to answer international concerns at the meeting with Western powers next week.
The emboldened calls to action came after Russia finally conceded that sanctions may be inevitable, after intense lobbying by the Americans.
President Ahmadinejad was among the leaders to make a speech at the UN General Assembly this week and he used the opportunity to repeat claims that Iran required only a civilian nuclear programme for the production of energy.
The Islamic Republic insists that it has the right to generate fuel for what it says will be a nationwide chain of nuclear reactors.
Iranian opposition leaders disputed that claim last night, suggesting that Mr Ahmadinejad was working to build nuclear warheads at two previously unknown weapons factories.