The Air France plane that went missing over the Atlantic this week was likely to have broken up in mid-air, experts said yesterday.

The vast area over which debris has been found suggested there was an explosion while the aircraft was in flight.

Experts said the "wide dispersion of wreckage discovered suggests that the Airbus exploded at high altitude".

Terrorism has not been ruled out but they said the most likely scenario was that the break-up was caused by massive depressurisation inside the plane.

If depressurisation had occurred at high altitude, passengers would have almost certainly fallen unconscious instantly and may have been unaware of their fate.

Quick

Professor Philippe Juvin, of Beaujon hospital, west of Paris, said: "It would have been as quick as the moment when one falls asleep."

Depressurisation can be caused by failure of the pressure control system, reduced cabin air inflow, or structural failure -- such as an open door, a cracked window, or a hole caused by a bomb.

The structure can also disintegrate if the G-forces during a dive are more than the plane can cope with.

Investigators will examine a bomb threat made against a flight from Buenos Aires to Paris just days before Flight 447 disappeared.

One unnamed Air France pilot suggested that a bomb could "very well" be the cause of the crash.

He said: "One can very well imagine that a bomb caused the aircraft's depressurisation and that the plane took time to break up. It could just as well have been a big bomb that blew up the entire plane, which would explain why the aircraft didn't have time to send an alert signal."

Crash investigators said they were "not optimistic" about retrieving the plane's black boxes, despite confirmation that debris spotted 400 miles off Brazil's coast came from the missing plane.

Difficult

Paul-Louis Arslanian, of the French civil aviation ministry, said it would be very difficult to recover the flight data recorders because of the depth of the ocean -- up to 10,000ft -- and its rugged floor.

"The investigation will not be easy ... but we are not giving up," he said.

A French mini-submarine will arrive at the area next week.

The slick from the fuel of Air France flight AF447 is seen from the window of a Brazilian Air Force plane patroling the crash area in the open Atlantic Ocean some 745 miles northeast of Recife yesterday