Blair: We've 48 hours to save sailors

The Prime Minister's comments came as a new picture of the hostages was released by an Iranian news agency. He told Iran the "door is open" to a diplomatic deal.

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This picture, apparently a still taken from a video, showed six sailors sitting in blue and red tracksuits on a carpet in a room with what appears to be a little bowl of nuts standing in the middle of the group

Mr Blair was responding to comments by a key player in the Iranian government who had raised hopes by referring to secret talks going on between his foreign ministry and the Foreign Office.

Iran's chief negotiator Dr Ali Larijani stressed such moves were at an early stage but added: "If they continue on this path, they can change the situation logically."

Referring to Dr Larijani's comments, Mr Blair said: "The next 48 hours will be fairly critical."

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Hostage Arthur Batchelor poses with Wayne Rooney at a navy dinner

He said Dr Larijani had offered the prospect of an end to the crisis. "If they want to resolve this in a diplomatic way the door is open," said Mr Blair.

Fifteen Royal Marines and sailors were seized as they patrolled the Shatt al Arab waterway in the Gulf 12 days ago.

Some have been shown on TV making "confessions" that they strayed into Iranian waters. There have been fears of Tehran staging a show trial.

Today's new picture, an apparent breach of the understanding that no more such photographs would be published, shows six sailors sitting in blue and red tracksuits on a carpet in a room. What appears to be a little bowl of nuts stands in the middle of the group.

The caption to the picture says: "British sailors are chatting and eating fruit, drinking coffee and playing chess. It seems that the sailors are satisfied with their situation, in which they are enjoying good conditions instead of working in a hard situation in the Persian Gulf."

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Some of the kidnapped sailors. Click enlarge for bigger picture

But Dr Larijani has stressed that his priority is to resolve the dispute by diplomatic means and without putting the captives on trial.

There was further optimism when Iran's firebrand president Mahmoud Ahmadinejad cancelled a press conference at which he was due to speak on the fate of the captives.

The decision to postpone the media briefing gives a breathing space for the diplomatic moves.

Britain's former ambassador to Iran, Sir Richard Dalton, said he was cautiously optimistic following Dr Larijani's comments. He said: "It looks as though both sides are looking for a way to solve the crisis and to do so in the not-too-distant future."

The UK insists the captives were taken from Iraqi, not Iranian, waters. Dr Larijani has suggested that he wants Britain to admit to its "mistake".

Britain may hold talks about how to avoid future clashes in the disputed waters between Iran and Iraq in which the 14 men and Leading Seaman Faye Turney were captured as they searched for smugglers.