The surprise release of 15 Royal Navy personnel on the orders of President Ahmadinejad was the result of a fierce debate within the Iranian regime rather than the product of negotiations with Britain.

When the Iranian leader suddenly announced that he was letting the British sailors and marines go, no one was more surprised than the officials involved in securing their freedom at Downing Street, the Foreign and Commonwealth Office and the Ministry of Defence.

“We had no idea they were going to be freed so soon,” one Whitehall source said.

In fact, Margaret Beckett, the Foreign Secretary, had been dreading the press conference by the notoriously hardline Iranian leader, fearing that his typically inflammatory comments could set back diplomatic efforts.

The Iranians did not reveal what had prompted them to make such a dramatic public climbdown, but the evidence suggests that it came after a battle between extremist and pragmatic elements in the leadership.

Tehran had made no secret of its intention to capture alive American or British military personnel, after their own officials were arrested by US forces in Iraq in January.

When the opportunity presented itself in the northern Gulf last month the Iranians successfully ambushed a poorly armed Royal Navy boarding party from HMS Cornwall. But having seized the 15 Britons there appeared to be confusion about what to do next.

The day after the incident, the United Nations Security Council passed a second resolution imposing sanctions against Iran for its controversial nuclear programme.

When the Iranians openly threatened to put their British captives on trial for “invading” Iran, Britain mobilised its allies abroad.

The UN Security Council issued a statement calling for the release of the captives. A tougher message went out from European Union leaders. Britain’s allies in the Middle East, such as Turkey, Saudi Arabia, Syria and Iraq, also weighed in on its behalf, warning the Iranians that their actions could provide the pretext for American military action. President Bush referred to the British captives as “hostages” and ruled out any swap for their release.

Far from winning international support for their defiant move against Britain, the Iranians now faced the prospect of being even more isolated than before. Their weak economy could be vulnerable to further international measures. The dispute with Britain threatened to drag on for weeks and might precipitate a military confrontation with American forces massing in the Gulf.

Extremists rejected any talk of a compromise and arranged a public demonstration on Sunday outside the British Embassy in Tehran, where they called for the captives to be tried and the British Ambassador to be expelled.

But behind the scenes Ali Larijani, the head of Iran’s Supreme National Security Council, was already pressing for a compromise. With the backing of Ayatollah Ali Khamenei, Iran’s Supreme leader, he toned down the rhetoric by sending a conciliatory message on Monday night in an interview he arranged on Channel 4 News promising that Iran wanted a peaceful settlement.

Then a press conference scheduled by President Ahmadinejad for Tuesday was mysteriously postponed without explanation. The first sign of a breakthrough came that night when Sir Nigel Sheinwald, Tony Blair’s foreign affairs adviser, spoke to Mr Larijani for 40 minutes. The talks were positive, but Downing Street did not expect that the captives would be freed in less than 24 hours.

For nearly two hours yesterday President Ahmadinejad remained true to form, condemning Britain’s interference in Iranian affairs and blaming the British for many of Iran’s ills. Only at the end did he announce the release.

For the first time in his 20-month presidency, Mr Ahmadinejad made a magnanimous gesture to the West. Whether or not he was forced to back down by the Supreme Leader may never be known. But it was obvious that he turned the occasion into a huge publicity coup.

For his hardline supporters, he spoke with characteristic defiance for 90 minutes against Tony Blair and George Bush. For the last few moments he showed generosity towards a group of young sailors and marines, including a young mother whom he insisted should never have been thousands of miles away from her family back in England.

What he said

From President Ahmadinejad’s speech yesterday

“On the occasion of the birth anniversary of the great Prophet of Islam, and on the occasion of Easter and Passover, I would like to announce that the great nation of Iran, while it is entitled to put the British military personnel on trial, has pardoned these 15 sailors and gives their release to the people of Britain as a gift”

“I would like Mr Blair’s Government not to punish the sailors for acknowledging and telling the truth”

“I thank the border guards who bravely protect our borders and also arrested the violators, and I grant them the bravery medal to their commander”

“We are sorry that British troops remain in Iraq and their sailors are being arrested in Iran”

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