Faye Turney, the lone woman prisoner

London, April 1: British ministers are preparing a compromise deal to allow Iran to save face and release its 15 British military captives by promising that the Royal Navy will never knowingly enter Iranian waters without permission.

The Sunday Telegraph has learnt of plans to send a Royal Navy captain or commodore to Tehran, as a special envoy of the UK government, to deliver a public assurance that officials hope will end the diplomatic standoff.

The move, which was discussed at a meeting of Whitehall’s Cobra crisis committee yesterday, came as Downing Street officials explicitly cautioned against hopes of a speedy outcome and said that families of the hostages should prepare for the “long haul”.

British Prime Minister, Tony Blair, and foreign secretary Margaret Beckett have been warned that the impasse may develop into a long-term standoff. Privately, officials are speculating that the crisis could continue for months.

The renewed search for a solution was given greater urgency when a senior Iranian official said that moves had begun to put the 15 British captives on trial.

Iran’s ambassador to Moscow, Gholamreza Ansari, announced: “Legal moves to determine the guilt of the British sailors have been launched.” In an interview with a Russian television channel, he said: “The legal process is going on and has to be completed and if they are found guilty they will face punishment.”

Iran’s President Mahmoud Ahmadinejad plans to make a formal statement on the crisis on Tuesday. Last night, he denounced Britain’s failure to apologise and decision to go to the UN: “This is not the legal and logical way.”

Beckett revealed that Britain has replied to a letter from the Iranian Embassy in London, sent on Thursday, which called on the government to acknowledge that the sailors had trespassed into Iranian waters and confirm that it would not happen again.

She said: “Everyone regrets that this position has arisen. What we want is a way out of it. We want it peacefully and we want it as soon as possible.”

Defence officials emphasised that they were not preparing to concede that the two British boats detained nine days ago were at fault. But one said: “We are quite prepared to give the Iranians a guarantee that we would never knowingly enter their waters without their permission, now or in the future.

“We are not apologising, nor are we saying that we entered their waters in the first place. But it may offer a route out of the crisis.”

Details of the strategy emerged as a former Falklands War commander expressed fury at how the sailors surrendered to Iranian gunboats without a fight. Maj. Gen. Julian Thompson called for a review of the UK navy’s rules of engagement, dictated by the UN, that they cannot open fire unless they are shot at first.