It is the first time since the end of the Cold War that fighters are being regularly scrambled from UK air bases to monitor aircraft in national airspace.

The operation to check up on planes is producing a bill running into tens of thousands of pounds as it costs between £7,000 and £9,000 for a Tornado to be in the air for an hour.

Air Chief Marshal Sir Glenn Torpey, the Chief of the Air Staff, said that since the 9/11 terrorist attacks in New York and Washington there had been at least one incident a month requiring an RAF fighter from a quick-reaction alert unit to intervene.

He told The Times: “The Tornados have been launching pretty regularly for any aircraft that appear to be behaving oddly: for instance, where airline pilots fail to communicate with flight control or take an unexpected route.”

He added: “We’re pretty acutely aware of the short time that we have to respond to these incidents. If there is any doubt at all about an aircraft, we launch the Tornados.”

Before the 9/11 attacks there were no interceptions of commercial airliners but the heightened terrorist threat now means that any deviation from a strict flight plan tends to trigger an interception by a Tornado. Four quick-reaction alert Tornado F3 planes are on permanent standby to investigate any plane that has not conformed precisely to accepted flight control procedures. Two are at RAF Marham in Norfolk and a further two at RAF Leuchars, the most northerly air defence station, in Fife, eastern Scotland. In addition, RAF fighter crews are kept in close proximity to royal flights in case of terror emergencies.

The two-seat supersonic fighter has a top speed of 1,500mph and can patrol for a long period at great distance from its base.

Next July the RAF’s new Eurofighter Typhoon combat aircraft will take over the role of guarding airspace from hijacked airlines in the southern approaches for the first time. They will operate from RAF Coningsby in Lincolnshire.

The RAF chief told The Times of an incident last month when the captain of a North West Airlines plane flying from Charles de Gaulle airport in Paris to Detroit in the United States asked to divert to Prestwick after a passenger became disruptive. The RAF was alerted and a decision taken to shadow the flight with a Tornado F3 that was already airborne. When the passenger became increasingly agitated the airline pilot was put on route to Prestwick, near Glasgow, and landed safely.

UK air defence has resumed its place as an important part of RAF responsibilities as the threat from international terrorism has increased.

The RAF chief said that there had been a long period of “zero” Quick Reaction Alert operations after the end of the Cold War when fighters were regularly scrambled to chase away potentially hostile Soviet aircraft.

Strict procedures are laid down for dealing with an emergency in British airspace. If an airline is known to have been hijacked by terrorists intending to use in a 9/11 type attack on the ground, the Prime Minister would be involved in giving authorisation to shoot it down, according to Whitehall sources.

The chain of command for authorising an interception and the shooting down of a commercial airline is short and always in political and not military hands.

Although the Government’s special emergency alert system was reviewed in the aftermath of the 9/11 attacks, it was found to be adequate to deal with a threatened terrorist attack.

Defence sources said although the Prime Minister would normally have to give personal authorisation for an airliner to be shot down, others are also authorised in the event of the Prime Minister being out of contact. The sources said that the other ministers were the Deputy Prime Minister, the Home Secretary and the Transport Secretary.

The decision, taken at a meeting of Cobra, the Cabinet Office emergency committee, would be based, not on the hijack itself, but on the perceived intent of the hijackers.

If a hijacked aeroplane was heading for London, the first warning would be received by the London joint area operations air traffic control centre at West Drayton — a military/civilian facility. Pilots have a “hijack” button in the cockpit whihc flashes straight to West Drayton.

The defence sources added that alarm bells would sound if an aeroplane started heading for an obvious target such as a nuclear power plant or other highly sensitive sites.

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