The British sailors and marines being held by Iran were ambushed at their most vulnerable moment, while climbing down the ladder of a merchant ship and trying to get into their bobbing inflatables.

Out of sight of their warship and without any helicopter cover, their only link to their commanders was a communications device beaming their position by satellite.

That went dead as they were captured. One theory is that it was thrown overboard to prevent the Iranians getting hold of the equipment and the information it contained.

The Ministry of Defence released the coordinates of the searched vessel yesterday to prove that the Iranian Revolutionary Guards made an unprovoked and improper attack in Iraqi waters.

The Iranians also blundered in diplomatic talks by giving the British their own compass reference for the place where they said the 14 men and one woman had been seized. When Britain plotted these on a map and pointed out that the spot was in Iraq’s maritime area, the Iranians came up with a new set of coordinates, putting the seizure in their own waters.

The speed and cunning shown by the Revolutionary Guards has raised suspicions that their action was premeditated. A senior military officer described it as “deliberate”.

It took only three minutes for the Iranians, moving at 40 knots, to move from their legitimate positions monitoring shipping in their waters to come alongside the British last Friday morning.

The sailors and marines from HMS Cornwall were in the Gulf, working under a United Nations mandate to protect Iraq from smuggling and threats to the oil industry, when an Indian-flagged vessel came under suspicion.

It was in shallow waters and the Cornwall was unable to go alongside without grounding. A boarding party jumped into two ribbed inflatable boats, or RIBs, and set out to investigate.

A helicopter hovered to observe the boarding but, after confirming that the Indian vessel was peaceful and friendly, returned to the ship. The Cornwall stayed in contact with the two launch boats via a communications link providing a GPS satellite position.

After the successful boarding of the innocent Indian vessel, the Britons began returning to their RIBs. At that moment one Iranian patrol vessel came alongside, adopting a friendly posture. As a second Iranian vessel arrived, the Revolutionary Guards turned aggressive.

HMS Cornwalllost communications with the launch boats and sent up the helicopter to investigate. The air crew watched as the small British inflatables were forced towards Iran. By now, up to four Iranian Revolutionary Guard vessels were swarming round the Britons.

Although the seizure has been widely linked to the taking of five Iranians by US forces in Iraq, Iranian diplomats have ruled this out. They say that there is no relation between the Britons’ seizure and any other bilateral, regional or international issue.

From the start, the Iranian Ambassador to London gave British diplomats a set of coordinates for the location of the confrontation.

Margaret Beckett, the Foreign Secretary, told the Iranian Foreign Minister that these compass points actually indicated a spot clearly in Iraqi waters. She tried to give Iran an exit route by suggesting that it might all be a misunderstanding that could be resolved by an immediate release of the captives.

On Sunday, the helicopter from HMS Cornwall flew back over the Indian vessel, which was still anchored and had drifted only slightly. A photograph was taken of an airman holding a GPS device. The coordinates on this picture, the MoD insists, prove that the Britons were comfortably within Iraqi waters when captured.

On Monday, Iran surprised Britain by coming up with a “corrected” set of coordinates. “The two Iranian positions are just under a nautical mile apart, 1,800 yards or so,” Vice-Admiral Charles Style, a Deputy Chief of the Defence Staff, said yesterday.

Mrs Beckett told the Iranian Foreign Minister that she could not accept the Iranians’ version of events. She told MPs in the House of Commons that it was “impossible to believe, given the seriousness of the incident, that the Iranians could have made such a mistake with the original coordinates, which after all they gave us over several days”.

Outgunned

— The two Iranian patrol ships that seized the Britons were equipped with rocket-propelled grenades and heavy machine guns, enough for a small sea battle. By contrast, the Britons go lightly armed on vessels they search in the Gulf. Each man is issued with a rifle or a pistol

— The Iranians struck at a vulnerable moment when the Britons were climbing down a ladder to jump into their inflatables

— The Royal Navy does train its men in the techniques needed to fight at just such a dangerous stage. “They had all the rights available to act in self-defence under law,” a senior military officer said. But they were in an “almost impossible position”

— A similar decision to hold fire was taken by the six Royal Marines and two sailors captured by Iran in 2004 in similar circumstances. Scott Fallon, a former marine, said they did think about shooting their way free but knew it would be hopeless. He told BBC Radio 4: “They had antiaircraft guns. We would have stood no chance”

Times Recommends

• Oprah Winfrey says goodbye to her talk show
• Focus on the big issues, not the bananas
• Václav Havel — symbol of national liberation