More than ten aircraft a week are climbing or descending to the wrong height in British airspace and putting passengers at risk of a mid-air collision, according to official figures.

The number of reported “level busts”, in which an aircraft deviates from its assigned height by more than 300ft, has more than doubled from 203 in 2003 to 566 last year. The scale of the problem raises doubts about the safety of the Government’s plan to allow the number of flights to double by 2030 in what are already the most overcrowded skies in Europe.

Ministers are planning to publish proposals in the autumn for a third runway at Heathrow and more intensive use of the existing two runways. These could result in another thousand flights a day over London.

National Air Traffic Services (Nats), which controlled the movement of 2.4 million flights last year, up almost 10 per cent in two years, is so concerned by the number of level busts that it has established a special programme to raise awareness of the risks among pilots and its controllers.

Aircraft are meant to remain a minimum of 1,000ft (305m) apart vertically and three miles horizontally. Most level busts do not result in near misses because there are no other aircraft in the area, but the projected increase in flights will make it much more likely that a mistake would place two aircraft on a collision course.

There are various safety mechanisms designed to ensure that pilots receive and act upon the right information from controllers. When a controller tells an aircraft to move to a certain height, the pilot must repeat the flight level over the radio so that the controller can be sure that he has been heard correctly.

The biggest cause of level busts, however, according to the Nats figures, is pilots correctly repeating the height but failing to comply with the instruction. This accounted for 27.9 per cent of level busts last year, compared with 19.4 per cent in 2004. The proportion caused by faults in setting the altimeter has doubled over the same period to 10 per cent. Other causes include pilots mishearing the original instruction or mistakenly believing that an instruction given to another aircraft was intended for them.

All commercial aircraft have to carry a back-up safety device, known as a Traffic Alert and Collision Avoidance System. This is meant to prevent collisions by detecting another aircraft at the same level and sounding an alarm. The systems on the two aircraft communicate with each other and tell one pilot to climb and the other to descend.

David Learmount, the safety editor of Flight International magazine, said that the system was not foolproof and depended on pilots reacting promptly to it. Failure by a Russian pilot to obey it resulted in a collision over Lake Constance on the Swiss-German border in July 2002, in which 71 people died, including 45 children.

The system can also fail to work if an aircraft’s transponder, which detects and responds to incoming signals, is not operating. Last September 154 people died in Brazil when a passenger plane hit a private jet on which the transponder was not working.

Mr Learmount said: “If you have higher congestion, the chances that a level bust will cause a collision are that much higher.”

A Nats spokesman said that the number of level busts had begun to fall in the first six months of this year and the number judged to be “safety-significant” had not increased in the past four years. He said that most of the increase in level busts was the result of Nats encouraging greater reporting of them.

He added that the recent introduction of “Mode S” radar had added another layer of safety by telling the controller not only the height of an aircraft but the level to which it was climbing or descending, allowing mistakes to be spotted more quickly.

Edward Lister, the leader of Wandsworth Borough Council and spokesman for the 2M group of 12 local authorities campaigning against Heathrow expansion, called for an inquiry into the increase in level busts. “Each level bust carries the potential for a mid-air collision,” he said. “Within the industry it is thought only around one in three incidents are actually reported.

“It is inconceivable that ministers could give the go-ahead to the expansion of Heathrow without a full investigation of the safety issues. I will be writing to the chairman of the Transport Select Committee [Gwyneth Dunwoody] urging her to take this up before the Government makes irrevocable decisions on expansion.”

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