The Roman Catholic Bishop of the Forces has appealed to the Supreme Leader of Iran, Ayatollah Ali Khamenei, to help secure the release of the 15 captured Royal Navy sailors and Royal Marines.

His appeal came as Tony Blair announced a possible breakthrough in diplomatic talks with Iran, although the Prime Minister admitted that the next 48 hours would be "fairly critical".

Bishop Tom Burns called on Khamenei, the country's spiritual leader and highest authority, to be generous with the captives.

He said Khamenie had "the unique opportunity to gain the world's respect" for Iran's Islamic laws and values.

"As one religious leader to another, I therefore ask him to show generosity by allowing one Navy to return the sailors and marines of another to their mother-ship.

"Britain will then be free to state that every care will be taken to avoid a similar situation in the future. Iran must know that such assurances will be readily given.

"This way ahead could allow both nations to break the impasse that currently
exists between them," he said.

He commended the Iranian government for treating the sailors well and said he was praying every day for them and for their families.

Mr Blair has told Iran on Tuesday that the way was open for diplomatic efforts but also warned that Britain would consider taking increasingly tough measures in order to secure the release of the sailors.

"All the way through this we've really had two tracks on this: one is to make sure Iran understands that the pressure is there available to us if this thing has to be hard and tough and long.

"We're not looking for confrontation over this and actually the most important thing is to get the people back safe and sound. And if they want to resolve this in a diplomatic way the door is open," Blair told a radio station in Scotland.

"The next 48 hours will be fairly critical," he said.

Relations between Britain and Iran have been difficult since the seizure of the sailors on March 23.