The RAF search and rescue team said that something catastrophic must have happened to the helicopter which crashed into the North Sea yesterday.

The crash has killed eleven people, including two men from Teesside.

Flight Lieutenant Paul Hopson - the pilot of the Sea King Mark 3A scrambled to search for survivors - said today: ''At this stage we cannot say what happened but all the indications are of a major mechanical failure.

''As far as we were aware, there was no May Day call and certainly no emergency beacons were activated, which suggest it happened very quickly.''

Co-pilot Flight Lieutenant Steve Murkin said it was unlikely to have been a controlled ditching and the crew would have had little time to escape.

He said: ''As more and more information came, it looked like it must have been something quite catastrophic. It was not a controlled ditching as we had hoped.

''It must have been something major because it sank quite quickly and nobody got out.

''If it is a controlled ditching, as soon as the helicopter hits the water, big flotation bags are inflated but there were none visible, so we do not know if they were activated.''

He added that the crew, including passengers, would have had survival and escape training and would have been wearing specialist gear, enabling them to survive in the water for hours.

In ordinary clothes, a person would be dead within an hour, he said.

The helicopter was scrambled at 7.50pm from Wattisham Airfield and arrived at the crash scene, some 30 miles north east of Great Yarmouth, at about 8.30pm yesterday.

On arrival, the crew saw a patch of oil and small debris covering an area just 200 yards across.

Boats which had arrived within seven minutes had already recovered five bodies out of the 11 people on board.

Flt Lt Murkins said the visibility was good and the water was calm so if there had been any survivors they would have been spotted easily.

''It was perfect conditions for searching. We could see everything on the surface and it was quite evident there was nobody on the surface to rescue. The biggest thing we saw was a seat cushion.

''Because there is always an outside chance of survivors, you keep looking. But within an hour it was pretty clear we were not looking for survivors in this case anyway.

''Given the speed at which the rescue agencies were on the scene, it is likely some of the bodies will be found in the wreckage.'' Following an intensive search along with 18 boats, the helicopter crew left just before midnight.

An investigation will be carried out by the Air Accident Investigation Board, part of the Civil Aviation Authority, said Flt Lt Hopson.

The wreckage will probably be recovered by specialist salvage units and form part of the investigation to find out exactly what happened, he said.