The body of the fourth victim of the Shetland helicopter crash has been recovered from the wreckage of the aircraft, Police Scotland said.

The three men and one woman died when the Super Puma L2 helicopter in which they were travelling crashed off as it ferried them from an oil rig in the North Sea.

The dead were named by police as Duncan Munro, 46, from Bishop Auckland; Sarah Darnley, 45, from Elgin; Gary McCrossan, 59, from Inverness; and George Allison, 57, from Winchester.

The bodies of two people were recovered from the sea on Saturday, while another died on the way to hospital, said rescue workers.

A fourth body was believed to be still in the wreckage. All 14 other passengers survived and were taken to hospital in Lerwick.

Police declared the crash a major incident.

The Super Puma L2 helicopter crashed about two miles west of Sumburgh Airport at 6.20pm on Friday.

The helicopter was carrying 16 passengers and two crew from the Borgsten Dolphin oil rig when it suffered a "catastrophic loss of power", said RNLI rescue co-ordinator Jim Nicholson.

The aircraft appeared to have "suddenly dropped into the sea without any opportunity to make a controlled landing", he said.

Lifeboat crews from Lerwick and Aith were joined by coastguard helicopters, police and the RAF in the search for survivors.

The ditched helicopter, which was operated by CHC for Total, was found broken into several pieces against rocks.

Coxswain Bruce Leask, of the Lerwick lifeboat, said two bodies were recovered after being spotted in the water by helicopter crew.

The wrecked helicopter had been towed off the rocks, and secured by rope chords in the shelter of Horse Island until a recovery vessel could reach it, said Leask.

One survivor, oil rig worker Sam Smith, told his mother the helicopter "rolled over" without warning before dropping into the sea.

Amanda Smith said she had spoken to her son by telephone as he recovered in hospital.

"He said it seemed to lose power and there was no time to brace, they just dropped into the sea," she said.

"He was by the window so he was able to escape that way as it rolled over.

"He said he had come off better than a lot of people. It didn't seem real, I would say two hours later it's just beginning to sink in."

'Major incident'

Michael Bull, whose son Samuel was among those rescued, said: "We understand he was on his way back from a rig and the helicopter lost power suddenly and immediately ditched into the water.

"He managed to escape straight away because he was right by an exit and I understand soon afterwards that the helicopter turned over."

A spokesman for helicopter operator CHC said the cause of the crash was unknown.

The company confirmed it had grounded its entire fleet of 17 Mark 2 Super Puma helicopters.

"The aircraft was on approach to Sumburgh Airport at approximately 6.20pm when contact was lost with air traffic control," he said.

"We can confirm there were 16 passengers on board, and two crew."

RNLI rescue co-ordinator Jim Nicholson said he believed the incident was caused by mechanical failure rather than bad weather.

"There was a fresh wind, not overly strong, visibility is not particularly good and it was misty in the area but I doubt if that would have had any impact on causing whatever happened to the helicopter," he said.

Sumburgh Airport was closed to allow emergency services to deal with the ongoing incident.

Scotland's First Minister Alex Salmond said: "Our thoughts at this difficult time are with the families, friends and colleagues of those who lost their lives in this tragic incident."

Last year, two helicopters ditched in the North Sea, with both incidents attributed to gearbox problems.

Pat Rafferty, of the Unite union, called for an investigation.

"This is an absolute tragedy," he said.

"This is the fifth major incident in the last four years involving Super Puma helicopters in the UK offshore industry and the second resulting in fatalities.

"It's unacceptable and it can't go on. A full investigation must now take place and the industry's helicopter operators must use every means at their disposal to demonstrate that its fleet is fit for purpose."