Reports from the UK say the helicopter ditched in the North Sea around 25 miles off Great Yarmouth, on the east coast. The cause is unknown, although speculation is centering on mechanical failure considering the good weather conditions at the time.

It had been heading to the Santa Fe Monarch drilling rig, after it had earlier landed at the Clipper platform, on the field of the same name.

It was on this leg of the journey that the helicopter crashed.

Two crew members and nine rig workers were on board the Sikorsky S76, UK news services have reported.

The helicopter was owned by the firm Bristow and is believed to have been chartered by Shell Expro.

Bristows said the helicopter was fully fitted with safety equipment including flotation bags to hold it afloat long enough for people to get out, depending on conditions.

It quoted a spokesman for RAF Kinloss, which is co-ordinating rescue efforts, as saying five bodies had been recovered. An RAF Sea King helicopter and numerous boats are searching for the others. The recovered bodies were taken to the Santa Fe Monarch rig.

Shell said it had established an emergency control centre in Lowestoft, Suffolk.