Malaysia’s prime minister, Najib Razak, says he has confirmation that the aircraft debris found on Reunion island last week is from the missing Malaysia Airlines flight MH370.

“Today, 515 days since the plane disappeared, it is with a heavy heart that I must tell you that an international team of experts have conclusively confirmed that the aircraft debris found on Reunion island is indeed from MH370,” Najib said at a press conference at the Putra World Trade Centre in Kuala Lumpur.

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

The Malaysia Airlines flight went missing on March 8, 2014. It disappeared from radar contact while en route from Kuala Lumpur to Beijing with 227 passengers and 12 crew on board.

In Paris, the deputy French prosecutor, Serge Mackowiak, was more cautious, saying that there were very strong reasons to presume that the flaperon discovered on the Saint-André beach is from the missing plane.

No other Boeing 777s are reported to have crashed in the region.

Mackowiak said confirmation that the flaperon is from MH370 would only come after further tests on the wreckage, which would begin today (Thursday).

The prosecutor said that, because of the flaperon’s technical characteristics (its colour and the structure of its joints), representatives of the constructor Boeing had confirmed that it came from a Boeing 777.

The flaperon, which was found by a local council worker, Johnny Begue, was flown to Paris, and was then taken to the Direction générale de l’armement (General Directorate for Armament) near Toulouse for testing.

Malaysia Airlines said in a statement that the flaperon had been confirmed to be from Flight 370.

“Malaysia Airlines would like to sincerely convey our deepest sorrow to the families and friends of the passengers onboard Flight MH370 on the news that the flaperon found on Reunion island on 29 July was indeed from Flight MH370,” the company said.

There had been confirmation from the French authorities, the Malaysian investigation team, a technical representative from China and from the Australian Transportation Safety Bureau, the statement added.

“Family members of passengers and crew have already been informed and we extend our deepest sympathies to those affected,” the company said.

The statement added: “This is indeed a major breakthrough for us in resolving the disappearance of MH370. We expect and hope that there would be more objects to be found which would be able to help resolve this mystery.”

The remains of a suitcase also found on the beach close to where the aircraft debris was discovered are due to be subjected to forensic analysis in France.

Johnny Begue with the suitcase found on Saint-André beach.

Najib Razak said the burden and uncertainty faced by the relatives on board MH370 had been unspeakable. “It is my hope,’ he said, “that this confirmation, however tragic and painful, will at least bring certainty to the families and loved ones of the 239 people onboard MH370. They have our deepest sympathy and prayers.”

Najib said after the disappearance of MH370 that there was a “high degree of certainty” that someone on board the aircraft deliberately disabled its Aircraft Communications Addressing and Reporting System (ACARS), which transmits information about an plane’s engine health, and had also switched off the transponder, which transmits such details as altitude, speed, and location.

The prime minister said at that time that MH370’s movements were “consistent with deliberate action by someone on the plane”. The plane did turn around after take-off, and flew on well after its apparent disappearance, he said.

In January, the director-general of Malaysia’s Department of Civil Aviation, Azharuddin Abdul Rahman, officially declared the disappearance of Flight MH370 to be an accident and said that all 239 passengers and crew on board were presumed to have died.

He said the underwater search in the southern Indian Ocean was continuing, but after 327 days, and based on available data, survivability in the defined area was “highly unlikely”.

There are relatives of those on board MH370 who point out that the discovery of the flaperon and the confirmation by Najib Razak that the wreckage is from MH370 is not enough to lay the matter of what happened to the missing plane to rest.

There are those, including Malaysia’s former prime minister, Mahathir Mohamad, who say that the fate of the plane has been concealed.

Sarah Bajc, the girlfriend of passenger Philip Wood, has claimed from the beginning that there has been a conspiracy. She told the Malay Mail Online that the official story – that MH370 flew on for about six hours after it disappeared and went down about 2,000 kilometres off the western coast of Australia – had so many holes in it that it was impossible to believe.

“I hear all the theories,’ she was quoted as saying. “Some of them are absolutely crazy, but most of them are more believable to me than the official story.”

A tearful Bajc told CNN after the flaperon was found: “If ultimately this is a piece of the wing then that little thread of hope that I have been holding on to will have to break and reality will have to take over, but, up until now, I and most of the family members have continued to believe that, until we have a body, we can’t give up hoping that they’ll still come back.”

There has been speculation that MH370 was shot down by the United States military when it was en route to Diego Garcia, an atoll in the Indian Ocean that is owned by the British and is home to a major US military base.

This is a theory that has been put forward by the former director of the French airline Proteus, Marc Dugain. He has suggested that US military personnel may have shot down MH370 over the Indian Ocean to prevent it being used to attack the Diego Garcia base.

Dugain also speculated that the plane may have been forced to divert from its flight path because of remote hacking or an on-board fire.

He pointed to the testimony of residents of the Maldives who said they saw an airliner travelling towards Diego Garcia on March 8, but whose claims were dismissed.

According to France Inter radio, Dugain said he had been warned off delving too deeply into the fate of flight MH370 by a British intelligence officer who reportedly told him that he was taking risks.

After further testing, investigators may be able to say more about what happened to MH370, but the plane’s black boxes, which could reveal some of what occurred in the cockpit of the plane, and provide vital technical information about the flight, have never been found.

If the flaperon discovered on Reunion island is indeed from MH370, some theories – such as the suggestion that the plane landed at the Yubileyniy airstrip in Kazakhstan – may now finally be discarded.

The mystery of what caused the disappearance of MH370 is, however, still a very long way from being solved.