Is there an armed person on board?

Two years ago today, US security officials grounded an Air France jet at Paris Charles de Gaulle airport. It was preparing to depart for Los Angeles. A check of the passenger manifest had apparently revealed some of the names that raised suspicions. The flight was cancelled. Two years earlier, Richard Reid, the "shoe bomber", had attempted to blow up an American Airlines flight from Paris to Miami.

In the weeks following the Air France cancellation, many other transatlantic flights were disrupted on the orders of the US Department for Homeland Security . On New Year's Eve 2003, British Airways' flight 223 from Heathrow to Washington was given Air Force fighter escort for its landing at Dulles airport due to fears that terrorists were on board. The same flight was subsequently delayed and cancelled on five separate occasions; eventually, BA renumbered the flight.

These events sparked a debate about the deployment of armed "sky marshals" on British aircraft

At present, armed security officers, whose role is to protect aircraft against hijacking, are deployed on flights operated by the Israeli airline El Al. In the US, the Federal Air Marshals programme is well established, with armed officers travelling on a random selection of flights. On 7 December, air marshals shot and killed a passenger named Rigoberto Alpizar aboard American Airlines flight 924 on the ground at Miami airport. He was alleged to have claimed to have a bomb.

So, what are the chances that your flight is carrying a sky marshal? In 2002, the Department for Transport announced a programme to train armed police officers for use on British-registered aircraft; two years ago, the Department for Homeland Security said airlines flying to, from or over the US would, in certain cases, be required to carry armed guards.

British flight crew and passengers have expressed alarm at the prospect of travelling on an aircraft with at least one armed person aboard: "Guns and aircraft do not mix", says Jim McAuslan, general secretary of the British Air Line Pilots Association. His union offers advice to members on refusing to fly with a sky marshal on board.

One senior aviation source said most British captains would never operate a flight on which anyone was armed, because of the danger to the aircraft if the fuselage is punctured.

The research undertaken by The Independent suggests that the UK government is maintaining a pretence that sky marshals are operating in order to appease the US authorities.

The Department for Transport has agreed that the airline captain will be informed if a sky marshal is aboard. Security sources regard it as highly unlikely that any intra-European flights would carry sky marshals; among long-haul departures, only US-bound flights are seen as likely to carry armed officers.

As with all security matters, the airlines are tight-lipped. British Airways says that if any flight were considered at risk, the airline would simply not operate it.

Additional research by Vanessa Webb