Debris found on an Indian Ocean island last week is from MH370, Malaysia’s prime minister has said, confirming for the first time that the plane which mysteriously disappeared 17 months ago had crashed.

Najib Razak’s announcement in Kuala Lumpur, yesterday, seemingly ended an agonising wait for families of the 239 passengers and crew of the Malaysia Airlines flight who had demanded concrete proof of what happened to their missing loves ones.

The piece of debris, a wing component called a flaperon, was found last week on a beach on the French island La Reunion, near Madagascar.

It was flown to the French city of Toulouse where it was examined yesterday by French and Malaysian technical experts, and representatives from Boeing to determine any link to MH370.

"Today, 515 days since the plane disappeared, it is with a very heavy heart that I must tell you that an international team of experts has conclusively confirmed that the aircraft debris found on Reunion Island is indeed from MH370,” Razak said.

"We now have physical evidence that, as I announced on 24th March last year, flight MH370 tragically ended in the southern Indian Ocean.”

But next-of-kin, investigators, and the aviation industry are still left with the vexing question of what caused the Malaysia Airlines Boeing 777 aircraft to inexplicably divert on a flight from Kuala Lumpur to Beijing on March 8, 2014.

The flight apparently veered out over the Indian Ocean, flying for hours after its communications and tracking systems were shut off, in what remains one of the biggest mysteries in the history of aviation.

Najib gave no indication that the analysis of the debris yielded any clues into the cause of the disappearance.

"I would like to assure all those affected by this tragedy that the government of Malaysia is committed to do everything within our means to find out the truth of what happened,” he said.

"MH370’s disappearance marked us as a nation. We mourn with you, as a nation.”

Many relatives accuse Malaysia’s government and the airline of a bungled response to the disaster, possible cover-up and insensitive treatment of families, charges that are vehemently denied.

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Yesterday, French prosecutor Serge Mackowiak later confirmed the wing fragment, known as a flaperon, was from a Boeing 777 - the same make and model as the missing Malaysian airliner. He said the results of initial tests showed there were ‘very strong indications’ the flaperon was from flight MH370. He said confirmation would come after further tests on the fragment, which would begin today.

Investigators will try to confirm that the part comes from the missing Boeing 777 and glean whatever clues they can about the cause of the crash, experts said. But the question of why the aircraft vanished may be clearer only when the main debris field is found, and its flight data recorder and cockpit voice recorder have been recovered.

The airliner is believed to have crashed in the southern Indian Ocean, about 3,700 kilometres from Reunion.