An investigation into a North Sea helicopter crash which killed four oil workers has so far found no evidence of technical failure.

Three men and a woman died when a Super Puma plunged into the sea off Shetland on August 23. There were 14 survivors.

"To date, no evidence of a causal technical failure has been identified; however, detailed examination of the CVFDR (combined voice and flight data recorder) data and the helicopter wreckage is continuing," the Air Accidents Investigation Branch said in a report.

The report backs a statement by the Civil Aviation Authority (CAA) last week which said that information gathered so far did not suggest a technical problem was to blame.

The helicopter was travelling from the Borgsten Dolphin support vessel when it came down en route to Sumburgh Airport with 16 passengers and two crew on board.

The victims have been named as Duncan Munro, 46, from Bishop Auckland, County Durham; George Allison, 57, from Winchester, Hampshire; Sarah Darnley, 45, from Elgin, Moray; and 59-year-old Gary McCrossan, from Inverness.

The AAIB said that both the helicopter's engines were giving out power when the aircraft hit the water.

It said that data downloaded from the black box on Sunday showed that the flight approach of the Super Puma was normal until three miles from the airport. At two miles it was below its correct path.

The report said: "The rate of descent remained constant for a period, before increasing rapidly.

"Shortly thereafter the helicopter, which was intact, struck the sea in a near level pitch attitude with a slight right bank.

"Both engines were delivering power until impact."

All North Sea Super Puma flights were suspended in the aftermath of the crash but operators later took the decision to resume service.

The L2 type involved in the accident was initially reintroduced for non-passenger operations such as training flights, but Bristow Helicopters began passenger flights again on Monday.

The crash was the fifth incident involving Super Pumas in the North Sea since 2009.

In April 2009 an L2 went down north east of Peterhead on its return from a BP platform, killing all 14 passengers and two crew on board.

A fatal accident inquiry into the 2009 crash is expected to begin in Aberdeen next January.

Chancellor George Osborne travelled on board a Super Puma to visit the Montrose platform off Aberdeen yesterday.

He delivered a speech at the Offshore Europe conference before visiting the platform with representatives from Talisman Sinopec.

Pat Rafferty, Scottish secretary of the Unite union, said: "The AAIB report does nothing to alleviate offshore workers fears about the safety of the Super Puma fleet, nor does it provide any further detail into why four people could not escape the crash with their lives.

"Unite believes the industry and authorities are missing the point. It's not just about whether the Super Puma fleet is airworthy, more urgently it's a question of whether they are fit for the safe transportation of people to and from offshore installations.

"We reiterate our call not to delay on the immediate steps the industry can take now regarding offshore helicopter safety such as reducing passenger capacity, seat re-configuration and re-calibrating survival training models in order to maximise the safety of workers.

"Confidence remains shattered as many questions remain unanswered and we simply can't afford to wait on the AAIB final report - which could take months - to start the process of urgent reform in offshore health and safety."