The crew of the Portland coastguard aircraft made an emergency landing at the Royal Marines' barracks in Poole, Dorset after spotting the fire during a rescue search.

The fire broke out on the Sikorsky S-61N helicopter, which was out looking for a missing man on 15 July.

The Department of Transport's Air Accidents Investigation Branch has found a problem with a bearing caused the engine to catch fire.

Further investigation

It praised the actions of the four-strong crew and the commander who landed the helicopter.

But further investigations are to be carried out into why one of the main drive shafts had started to "run eccentrically".

The report pointed out that a similar accident had destroyed another S-61 helicopter in 1988.

A spokesman for Bristow Helicopters, who leased the aircraft to the coastguard, said it had an excellent safety record and maintenance was carried out "to the highest possible standard".

The spokesman said: "We know how this happened, what we don't know at the moment is why it happened.

"We are working closely with the Air Accidents Investigation Branch to find out everything we can about what happened and any necessary action will be taken."