The Royal Navy has launched an inquiry after sunbathers narrowly escaped injury when pieces of fuselage fell off its new multi-million-pound Merlin helicopter.

The aircraft, said by experts to be a world-beater, was on its way from an air show in Belgium to the naval air station at Culdrose, Cornwall, when debris fell 500ft on to Worthing beach.

Lifeguards manning Worthing Council's safety boat watched in amazement as the bits started falling off.

They recovered two chunks of plastic which when fitted together measured 3ft long by 18ins wide.

Mike Colliss, the council's foreshore manager, said: "We had the safety boat out doing routine patrolling when the helicopter passed. When it got to Goring the safety boat called us and said bits were falling from it."

While Mr Colliss called the coastguard base at Lee-on-Solent, lifeguards Dave Winstone and Martin Blaker recovered the pieces, which had serial numbers on them, from the beach.

Mr Winstone said the debris fell about 15 yards from people sitting on the shingle, adding: "I think they were a bit surprised."

Mr Colliss said: "They brought the pieces back to the beach office. They fit like a jigsaw puzzle. It looks like a cover of some sort, made of a very lightweight composite material. It's got a rubber seal all the way around and two fasteners that obviously didn't work."

Doreen George, spokeswoman for the air station at Culdrose, said: "The piece that fell off has been identified as a flotation gear cover. The aircraft is fitted with flotation bags which inflate when it lands on the water."

She said the pilot was unaware the cover had fallen off until alerted by air traffic controllers at Bournemouth airport.

He then made a precautionary landing at Bournemouth, inspected the Merlin, which had four people on board, and took off again.

Ms George said: "There will be an internal investigation. The fortunate thing is nobody was injured on the ground and no damage was done. The aircraft is still airworthy."

A Royal Navy spokesman said: "The Merlin is currently being introduced into service. It's a super new aircraft, and will replace the Sea King."

The Navy and RAF have ordered 66 of the Anglo-Italian Merlins, which will cost a total of £4.65 billion.

The helicopter is designed to operate with a three-man crew in all weathers from the flight decks of both large and small ships.

It can carry up to four homing torpedoes or depth charges, for use against submarines.

It can also be used for casualty evacuation, troop carrying, and search and rescue.