GORDON BROWN joined the US and France yesterday in demanding Iran give up its nuclear programme following the dramatic disclosure that Tehran is building another secret nuclear facility.

At a joint statement at the G20 summit in Pittsburgh, Mr Brown, President Barack Obama and President Nicolas Sarkozy warned Mahmoud Ahmadinejad’s regime that it must come clean about its programme or face international sanctions.

Mr Brown said the discovery of the covert site, buried deep in the mountainside near the holy city of Qom, had shocked and angered the international community.

“Confronted by serial deception over many years, the international community has no choice but to draw a line in the sand,” he said. “We will not let this matter rest.”

Mr Obama said: “The Iranian government must now demonstrate through deeds its peaceful intentions or be held accountable to international standards and international law.”

Mr Sarkozy warned: “We were already in a very severe confidence crisis. We are now faced with a challenge made to the entire international community.”

Their intervention set the scene for a showdown next week in Geneva when Iranian officials meet representatives of the E3+3 group of Britain, France and Germany, as well as the US, Russia and China.

Mr Obama said Iran would need to take concrete steps to create confidence and transparency in its nuclear programme.

The three leaders made clear that failure by the Iranians to meet their demands would see fresh impetus for tough new sanctions on the regime.

Significantly, Russia – which has previously resisted pressure for sanctions – said last night that it found the latest disclosures about Iran’s secret facilities “disturbing”.

The facility, which is thought to be still under construction, was originally discovered by British, US and French intelligence agencies three years ago.

Western diplomatic sources said it included an underground chamber big enough to hold 3,000 centrifuges capable of producing sufficient highly-enriched uranium to build one nuclear bomb every year.

The number of centrifuges was seen by intelligence analysts as highly significant as it was too big to be a pilot facility – as the Iranians claimed – and too small for a civil plant capable of powering a whole town.

The British, Americans, and French presented the details of their findings to the International Atomic Energy Authority (IAEA) – the world nuclear watchdog – in Vienna yesterday.

The Iranians leaked the details in an apparent bid to preempt yesterday’s joint statement.

Neither Russia nor China was briefed in advance about the disclosures and were said to be digesting the information.

Mr Ahmadinejad said last night that nuclear weapons were the arms of the last century and called for global nuclear disarmament.

He also said his country had complied with IAEA rules.

Speaking at a press conference, the Iranian leader said the new facility would not be operational for 18 months so he had not violated any requirements.

Mr Ahmadinejad dodged a question about whether Iran had sufficient enriched uranium to manufacture a nuclear weapon but said Tehran rejects such armaments as “inhumane”.

■ Gordon Brown said measures expected to be agreed yesterday at the G20 summit in Pittsburgh under the chairmanship of Barack Obama should see the world economy grow by a healthy three per cent next year.