It is believed the phantom aircraft, which appear over the North Sea, are caused by screens showing incorrect radar data.

It is the latest glitch to hit the �623m centre in Hampshire since it opened in January.

A spokesman for National Air Traffic Services (Nats) insisted safety was not being compromised by the non-existent planes.

Staff warned

The Nats spokesman said: "The ghosting problem has affected only a few screens and only affects an area of the North Sea.

"Safety has not been compromised, but, nevertheless, we felt it right to issue a notice to employees."

Nats has now warned controllers at Swanwick of the problem and told them to keep aircraft further apart than normal if the phenomenon occurs.

The problem is only one of a number encountered by the multi-million pound centre.

Recently controllers said they were concerned they might misread the height of planes because of difficulty in reading numbers on their screens.

Controllers have said the latest "ghost" problem did not occur at their centre in West Drayton, west London - the centre that Swanwick is replacing.