Iran has revealed the existence of a second uranium enrichment plant today, officials said.

The news came as Tehran was handed an ultimatum yesterday to agree to end its nuclear programme in the next two weeks or face harsh economic sanctions.

Officials say Iran's announcement was in a letter sent Monday to International Atomic Energy Agency chief Mohamed ElBaradei.

Iran is under three sets of U.N. Security Council sanctions for refusing to freeze enrichment, which can make both nuclear fuel and warhead material. It had previously acknowledged having only one such facility.

The government officials - one speaking from his European capital outside Vienna, the other a diplomat in Vienna - demanded anonymity today because their information was confidential.

Britain and five other major powers have joined forces to call for a ‘serious response’ from the regime in Tehran by October 1, or swinging economic, business and energy sanctions will be imposed.

Prime Minister Gordon Brown said the world was drawing a ‘line in the sand’ and was no longer prepared to tolerate Iran ‘misleading’ the world about its nuclear intentions.

Mr Brown said the issue would now be ‘coming to a head’ at a meeting in Istanbul next month at which Britain, the US, France, Germany, Russia and China will ask the Islamic regime for answers.

The Prime Minister said Iran now had the choice of giving up its work on uranium enrichment - a key step in building a nuclear weapon - and accepting international assistance with its civil nuclear programme or face international isolation.

'We are prepared to take far tougher economic and financial sanctions. We are prepared to join the international community in stepping up these sanctions,' he said.

'The strongest possible message is going out to Iran about how we see their behaviour and what we plan to do for the future.'

Significantly, Russia – long a close ally of Iran – has now said that it is prepared to consider sanctions.

While the Chinese have still to be persuaded, Mr Brown made clear that the three European countries and America are prepared to impose their own sanctions.

The Security Council yesterday approved an American resolution committing all nations to work for a nuclear free world. Russia and China signed up to the plan giving it greater force.

It calls for greater global efforts to prevent the spread of nuclear weapons, promote disarmament and ‘reduce the risk of nuclear terrorism.’

The document pulled together other resolutions on nuclear weapons into a single document and will add force to a diplomatic push to agree a new non-proliferation treaty next Spring.

That is expected to call for non nuclear states to abandon their ambitions in return for the nine nuclear states agreeing to cut the size of their arsenals.

Today’s resolution also calls for all nuclear materials to be ‘locked down’ within four years and calls for the Security Council to act if they fall into the hands of terrorists.

Mr Obama said: ‘The historic resolution we just adopted enshrines our shared commitment to a goal of a world without nuclear weapons. And it brings Security Council agreement on a broad framework for action to reduce nuclear dangers as we work toward that goal.’

But the more immediate problem confronting world leaders is how to prevent Iran developing nuclear weapons.

A Downing Street spokesman said: ‘The meeting on October 1st is a vital moment for Iran to address the international community’s concerns about its nuclear programme

We are clear that we want to engage with Iran and we hope that Iran will take this opportunity but equally we will not stand by when countries breach their responsibilities.

French President Nicolas Sarkozy added: ‘If we have the courage to affirm and impose sanctions on those who violate resolutions of the Security Council we will be lending credibility to our commitment to a world with fewer nuclear weapons and ultimately with no nuclear weapons.’

There has been little sign of compromise from Iran during the annual United Nations general assembly.

British diplomats walked out during a speech by President Mahmoud Ahmadinejad over 'anti-Semitic' remarks by the Iranian leader on Wednesday night

Mr Brown dodged another confrontation with a dictator after Libyan hardman Colonel Gaddafi failed to turn up for the Security Council meeting. British diplomats had been concerned that Mr Brown would be photographed shaking Gaddafi’s hand.

Instead, Col Gaddafi was speaking at the Council on Foreign Relations think tank in New York, where he warned against military action against Iran.

Insisting that Tripoli 'does not support Iran having nuclear weapons', Col Gaddafi questioned who would have the right to take military action against the country.

Hesaid action against Iran could set a dangerous precedent, noting that other countries including India, Pakistan, China, Russia, the United States and Israel have - or in Israel's case are assumed to have - atomic weapons.

'All of them have nuclear bombs. Why not take military action against them?' he said.

No country believes Iran yet has a usable nuclear weapon and experts differ over when it might develop one.

Iran itself strongly denies it it trying to build atomic arms and says its nuclear program is for peaceful purposes.

But Israel says it is in serious danger from Iran, whose president has called for the elimination of the Jewish state.

There has long been speculation Israel might launch a pre-emptive strike against Iran's nuclear facilities.

Yesterday, Israeli President Benjamin Netanyahu said a nuclear Iran 'should not be allowed to happen'.

In response, Iran  said it was open to talks but 'illegal demands' must be dropped.

After the Security Council meeting Mr Brown co-chaired a meeting with President Obama to secure greater support from Pakistan for taking on the Taliban in Afghanistan.

He then flew to Pittsburgh for the G20 summit tomorrow.

Mr Brown hopes to be able to secure a global deal on bankers bonuses that would lead to pay restraint in the City. But the plan to be agreed by the G20 leaders will not include a cap on individual payouts.

The Prime Minister hopes to win agreement from other leaders to beef up the G20 and make it the dominant international institution for coordinating economic policies.

By Daily Mail