MYSTERY last night surrounded the whereabouts of aerial footage taken of Manchester United stars in top-secret training sessions.

It remained unclear who authorised the filming in a specially converted Cessna light aircraft and whether the material is destined for this country or abroad.

The covert clips of players such as Wayne Rooney and Cristiano Ronaldo working on tactics, formation, free kicks and penalties during the astonishing spying mission could be invaluable to rival clubs.

Videos and stills of the training were driven from a Derbyshire airfield to Heston Services on the M4 - a mile from Heathrow airport - and handed to a mystery client in a Chrysler Grand Voyager with blacked-out windows.

He shook hands with the delivery man - one of two on board the Cessna 172 Skyhawk commissioned by a photographic agency - then left after a 30-second encounter.

Earlier, the Mirror watched as the plane - registration G-CBME - passed over United's high-security training complex at Carrington while manager Sir Alex Ferguson would have been putting his team through their paces. The sky blue-striped Cessna, owned by a company called Skytrax Aviation, swooped down to about 1,500ft to get the best shots.

It is believed three other spying mission have been carried out. The one we saw was last Wednesday morning.

Wayne Rooney, Rio Ferdinand and Paul Scholes were among the first-team squad training as the four-seater, single-engine plane - adapted to carry camera equipment - twice buzzed over them at 1,500ft between 11.45am and 11.50am.

A huge circular lens was clearly visible from the ground. A panel had been unscrewed from the belly of the plane providing a gap for both video and stills cameras to operate.

The plane skirted around the edge of the heavily guarded training ground before heading off into the distance.

An aviation source said last night: "G-CBME may look like a little old Cessna with a single propeller but it is anything but.

"It has a £100,000 engine which operates on jet fuel.

"This is very unusual and means that the plane is able to fly much more quietly which obviously does not alert people on the ground so much to its presence. It also has £50,000 worth of specialist equipment on board. It has been deliberately modified so it looks like a little sight-seeing pleasure plane but it is equipped for some of the best surveillance operations in Britain."

The plane had taken off in fierce cross winds from tiny Egginton airfield 60 miles away in Derbyshire 40 minutes earlier.

It arrived back at the grass airstrip 2.05pm with its potentially damaging cargo.

One of the men then jumped into an estate car and sped south and on to the M40.

When he reached the M25 he joined the M4 and drove towards London, arriving at Heston Services near Heathrow airport where he was flagged down by the client in the Chrysler.

An A4 sized silver coloured metal case was handed over. The contact smiled, patted the delivery man on the shoulder and then left.

United lead the Premiership and are still in the FA Cup and European Champions League competitions that could lead to the club's second Treble.

If it is proved another Premiership club has or ordered the footage there could be serious repercussions under Football Association rules on conduct.

The Mirror was alerted to the secret mission after a series of flight plans over Carrington were logged with Manchester air traffic control. Several non-standard flight numbers were given to controllers. An ATC source said: "We received a request for this NSF during the week beginning January 8.

"The pilot specifically asked for permission to film a small site to the edge of Carrington village and it immediately rang bells again because there have been three other very similar NSF requests in recent months.

"It was activated at 10.30am on Wednesday January 17. The plane was given permission to proceed."

The firm that carried out the surveillance is a respected aerial photographic agency called British Geographical.

An aviation source said: "I don't know much about them but I know they are highly regarded within the field of aviation photography."

Skytrax leased the Cessna out for £119 per hour to British Geographical specifically for this assignment - although they would not have known what was going to be filmed.

BG owner David Goddard said: "We never discuss who our clients are. We always fly under strict Civil Aviation Authority guidelines and always comply fully with local air traffic control."