A rotor blade in a helicopter that crashed into the North Sea off Norfolk killing 11 people, was suffering from fatigue, an official accident report has concluded.

The problem followed a "manufacturing anomaly" in the blade, made worse by a lightning strike in 1999, investigators have found.

The accident was caused when a spar in the rotor blade broke, said the report from the Air Accidents Investigation Branch.

The helicopter, owned by Norwich-based Bristows, was ferrying workers between the Shell gas field when it crashed 28 miles off the coast on 16 July, killing all on board.

The lightning strike in 1999 exploited the anomaly built into the blade at manufacture and damaged the blade spar

Accident report

There was "clear evidence of fatigue" in the rotor blade, the report states.

Neither the damage nor the fault was detected during an inspection by Sikorsky after the helicopter was hit by lightning, accident investigators said.

The helicopter was repaired and returned to service.

The "anomaly" would have been hidden from view during routine in-service blade inspections, the report concluded.

Since the accident, Sikorsky has removed from service any main motor rotor blade identified as having been damaged by a lightning strike.

The aircraft was flying to a drilling rig at a height of 320 feet when workers on the rig heard a loud bang, the report said.

Increase in vibration

Witnesses then saw the aircraft dive steeply into the sea.

The helicopter's black box showed that 4.5 minutes into the flight the crew discussed an increase in vibration but this did not cause any concern.

Both pilots and nine passengers died in the accident.

"It was clear the blade fracture had initiated the catastrophic event," the report said.

Manufacturing process

The report said the "anomaly" in the blade had occurred during the manufacturing process in March 1981.

"The lightning strike in 1999 exploited the anomaly built into the blade at manufacture and damaged the spar.

"If the anomaly was not detected during manufacture, there may be other blades where the anomaly is slightly different in nature and severity," the report concluded.