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He also accused the Iranians of having changed their story over the weekend after being told that the coordinates Tehran initially gave for the incident showed that the patrol boats were in Iraqi waters.

Minutes after the MoD press conference, the Prime Minister told the Commons that Britain was mobilising international support to show Iran how isolated it was. Mr Blair described the seizure as "completely unacceptable, wrong and illegal".

Responding to a question from David Cameron, the Conservative leader, about the rules of engagement the patrols were operating under, Mr Blair said that the sailors and Marines could have used force in self defence. But he was was quite satisfied that they had taken the right decision in not drawing their arms after being surrounded by six heavily armed Iranian Republic Guard vessels.

"If they had engaged in military combat at the stage, there would undoubtedly have been severe loss of life," he said.

Mr Blair added by the time the crew of HMS Cornwall realised that the 15 had been detained and a Lynx helicopter dispatched to find them, they were already in Iranian waters - making intervention that much more dangerous.

The vice-admiral said that far from being inside Iranian waters, the two boats were 1.7 nautical miles — almost two land miles — inside the Iraqi part of the Shatt al-Arab waterway, which forms the border between Iran and Iraq.

Their exact postition — 29' 50.36" N, 048' 43.08"E — was confirmed by a Global Positioning System (GPS) on one of the small patrol boats that was displayed on the Cornwall. It was confirmed on a subsequent fly-past of the site.

In a statement to the Commons, Mrs Beckett said that the Government had tried to deal with the crisis through "private, but robust diplomacy". When the Iranians' mistake over the coordinates had been established, she had suggested to her Iranian counterpart that the situation "could be easily resolved" by releasing the detained Britons.

But it was now clear that a change of tack was needed. Accordingly, Britain was mobilising diplomatic support to show Iran how isolated it was over the issue. The Foreign Secretary said that she had spoken to various world leaders, including the US Secretary of State, the Turkish Prime Minister and the foreign ministers of both Iraq and Iran.

Mrs Beckett said that the Government had "no doubts about the facts or the legitimacy of our requirements": that Tehran state where the detainees are being held, grant Britain consular access to them and give details of their release.

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