Eight British Marines detained by Iran for illegally straying into its waters could soon be freed, the Iranian media reported this evening.

Ali Reza Afshar, a senior member of the Iranian armed forces, told Iran's ISNA student news agency: "If the result of the interrogations of the British soldiers shows that they did not have bad intentions, they will be freed very soon."

Mr Afshar also said that Iranian security forces were interrogating the eight Marines "separately".

"We are continuing our investigation," he said, before adding that the Royal Marines had "thanked Iran for its kind hospitality".

Tehran-run Al-Alam television had reported earlier in the day that the eight men would be tried for illegally entering Iranian territorial waters, quoting Iranian military sources.

But in London the Prime Minister's official spokesman said that the British Government had not been told that the group would be prosecuted.

Mr Blair's spokesman said: "We have now been formally told by the Iranians that they are holding eight military personnel.

"The Foreign Secretary spoke to his opposite number in Tehran to raise concern about the detention and Foreign Minister Kharazzi said he would look personally into the case.

"We have asked for full details and for access to them and we will keep trying to establish the facts. We want to resolve this issue as quickly as possible."

The spokesman said that Britain had still not been told where the men were being held.

And he hinted that Mr Blair could become personally involved if the situation was not resolved, saying: "The way to resolve this is talking to the Iranians at all levels and see if we can resolve this as quickly as possible."

Al-Alam said that the Marines, who were detained yesterday and whose three boats were also seized by Iranian Revolutionary Guards, had already "confessed" to having entered Iranian waters.

The eight were part of a training team of 28 Royal Navy personnel who had been instructing Iraqi police in river patrol and coastal defence procedures.

Richard Dalton, the British Ambassador in Tehran, sought in vain yesterday to discover where the British men were being held and whether Iran was planning to provoke a wider diplomatic incident over their capture and the confiscation of their vessels.

Last week Britain was one of three European countries that drafted a toughly worded resolution, agreed by the members of the International Atomic Energy Agency, a UN agency, criticising Tehran for failing to come clean about its suspected nuclear weapons programme.

Britain has also been criticised over the occupation of holy Shia Muslim sites in Iraq, culminating in a series of violent demonstrations outside the British Embassy in Tehran.

Foreign and Commonwealth Office sources said that they thought it unlikely that the Iranians were holding the eight commandos in retaliation for last week's resolution. They were hoping that the incident - the first since the war in Iraq a year ago - was simply the work of an over-zealous local commander.

Royal Navy sources said that "no shots were fired" after a heavily armed Revolutionary Guard patrol boat crew ordered the British vessels to follow them to the coastline. The Marines agreed to be "guided" to the Iranian shore where they were taken away for interrogation. The sources said there appeared to have been a navigational error.

British and Iranian officers patrolling the Iran-Iraq frontier have been holding regular meetings since the war last year to avert precisely this type of incident. Until yesterday, relations, particularly with the regular Iranian Army as opposed to the pasdaran (the Revolutionary Guards), were said to be friendly and co-operative.

"It sounds like the work of an overzealous local commander in the Revolutionary Guards trying to make a point," Ali Ansari, an expert on Iran at Exeter University, said.

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