The growth in the number of small, private jets could cause a mid-air crash because a loophole in safety rules means that they lack collision- avoidance systems, according to Europe’s air traffic regulator.

This month the first of a new breed of very light jets, known as VLJs, began operating from small airports in Britain. More than 100 are due to be delivered to air taxi companies and private operators in Europe each year for the next decade, according to Eurocontrol, the Brussels-based body that oversees air traffic control. They will operate in controlled airspace at similar heights to airliners but will not have the advanced safety systems that prevent collisions.

The Traffic Alert and Collision Avoidance System (TCAS) is a cockpit device that monitors the airspace within 40 miles of the aircraft and informs pilots when there is any risk posed by another plane. If two aircraft continue on a collision course the system instructs one aircraft to climb and the other to descend. If one aircraft does not have the system or if it is not working properly, a collision may still occur.

In September 2006 154 passengers and crew onboard a Gol Airlines Boeing 737 died after an Embraer Legacy business jet sliced off its wing tip in Brazil. The 737 plunged 37,000ft (11,278m) into jungle. The Legacy was damaged but landed safely. Investigators found that the TCAS of the Legacy was not operating and could not correspond with the 737’s system.

Under international aviation safety rules all jets weighing more than 5.7 tonnes must have TCAS. VLJs typically weigh less than 4 tonnes and are therefore exempt.

Alex Hendriks, the deputy director of air traffic management strategies at Eurocontrol, told The Times: “TCAS is mandatory for airlines because of safety considerations. Why should we exclude a certain category of aircraft just because they are small?

“It is the same as saying that motorcycles are smaller than cars and therefore don’t have to have lights when they are ridden at night.” Mr Hendriks is leading a review about VLJs and the consequences for European airspace safety and capacity. One of the key questions is whether VLJs should lose their exemption from the TCAS requirement.

Mr Hendriks, a pilot who recently flew a Cessna Mustang light jet across the Atlantic, said: “We have already warned the VLJ operators that there may be additional regulations that will force them to have the full TCAS that airlines have. The risk will increase as the traffic increases.

“We don’t want to wait for a crash before we come up with a regulation.”

Any new regulation may not come into force until 2012 to allow operators time to install the equipment. It will cost £100,000 to equip each aircraft. The cramped cockpits may have to be redesigned to accommodate the equipment.

Jet Bird, which has ordered 100 Embraer Phenom VLJs and plans to start an air taxi service in Europe next year, has decided against fitting the system.

Stefan Vilner, its chief executive, said: “First of all it’s very costly and secondly it’s not required.”

He added: “I don’t think our customers would have a clue whether it was there or not. If you catch a Ryanair flight you don’t think about its TCAS, you assume safety is a given.”

Mr Vilner said that the risk of a collision would be minimised because Jet Bird would be operating away from the busiest sections of airspace.

David Kaminski-Morrow, of Flight International magazine, said: “Europe’s skies are already congested and VLJs will make them more so. I hope it won’t take a mid-air collision before VLJ operators start considering whether the safety investment is worth making for their passengers and their business.”

Eurocontrol is also concerned that VLJs will cause delays to scheduled flights because they operate at as little as half the speed of airliners.

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