Elite squads of anti-terrorist troops are preparing for a "strategy of last resort" as negotiations continue to bring the Stansted Airport hostage crisis to a peaceful conclusion.

Teams of SAS soldiers are ready to storm the aircraft at a moment's notice if they receive the go-ahead.

Metropolitan police chiefs and intelligence officials from MI5 are believed to be helping Essex police make strategy decisions on ending the crisis.

If the hijackers start killing hostages or police believe they are about to start killing them, the SAS squads will act immediately. They will only be used if the negotiations fail to spare the captive passengers.

Seconds count

The commandos - from the regiment's counter-revolutionary warfare troop known as The Team - will take the hijackers by total surprise, knowing they have only seconds in which to save the hostages.

Click here to see how the SAS might storm the plane

Intelligence chiefs will have been preparing by getting as much detailed information as possible from the passengers who have been freed.

They will be asking whereabouts in the plane the hijackers are sitting, who is in the cockpit and cabin, what weapons the group have, their emotional state and how they spend their time.

The security experts will also have access to highly technical surveillance equipment of the plane, a Boeing 727, together with computer programs giving realistic impressions of the jet's interior, and even real, disused 727s on which to practise.

Steven Simon, a former White House counter-terrorism expert, told BBC News Online: "They will go about it in a very careful and systematic way, because they want to reduce to a minimum the possibility of civilian casualties."

Aviation experts say the 727 is a small plane and would be relatively easy to storm.

Blinding grenades

The troops - equipped with body armour, fireproof overalls, combat boots and helmets - may well approach the plane along blind spots under the swept-back wings. They would then crawl, unseen, to the emergency exits.

Back-up groups could support them by vaulting through windows and other entry points.

The surprise element would be crucial: if the hijackers were alerted, they could become angry and violent.

Mr Simon, now assistant director of the International Institute for Strategic Studies, believes the squad would shut off the jet's power, blow in the doors, then use flash-bang grenades. The grenades would blind and deafen everyone on board for several seconds - except the troops themselves who would be protected.

"The commandos will be expert marksmen with special weapons and goggles to see through and hear through the effects of the grenades," he said.

Their equipment will allow them to talk to one another clearly during the storming operation.

They will be shooting to kill, he believes.

"I don't think the hijackers will have much opportunity to surrender once the doors are blown in. And there's no option to wound them - a wounded person in possession of a weapon is extremely dangerous.

Adrenalin-pumped

"Once the commandos start shooting, their aim is to kill.

"But I'm also fairly confident that if they don't have to storm the plane and kill these people, they won't."

There will be enough troops to ensure the plane is taken rapidly, but not so many that there is a risk of being killed by friendly fire, he says.

The men - all in their early 20s to early 30s - will be prepared for the danger emotionally as well as physically.

"These people are so adrenalin-pumped that they're ready for anything," says Mr Simon. "But they're also ready not to be used."

One former SAS commander keeping a watch on the Stansted crisis said the terrorists would have no escape.

"There are only two ways they are coming off that aircraft - either with their hands up or on stretchers. However, that is a long way off at the moment."

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