Part of the aircraft wing found on Reunion Island is from the missing MH370 plane, Malaysian Prime Minister Najib Razak has confirmed.
Mr Najib said international experts examining the debris in France had "conclusively confirmed" it was from the aircraft.
The Malaysia Airlines plane carrying 239 people veered off course from Kuala Lumpur to Beijing in March 2014.
The debris was found on the remote French Indian Ocean island a week ago.
The plane is believed to have crashed into the southern Indian Ocean - though no evidence had been found despite a massive search operation.
"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 [...] is indeed MH370," Mr Najib told reporters.
"We now have physical evidence that [...] flight MH370 tragically ended in the southern Indian Ocean," he added.
Mr Najib said he hoped the discovery "will at least bring certainty to the families" of the victims, saying the burden they had faced was "unspeakable".
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 initial tests showed there were "very strong indications" that it was from flight MH370. But he said confirmation would only come after further tests on the fragment, which would begin on Thursday.
"[Investigators] will try to do it as soon as possible in order to provide total and reliable information to the family of victims, who are on our minds at the moment," Mr Mackowiak added.
The Paris prosecutor failed to be as categorical as the Malaysian prime minister in asserting that the wing piece does come from MH370. All he said was that there are very strong reasons to presume that it is from the missing plane.
That does not mean that the prosecutor has any reason to doubt the prime minister's conclusion - simply that he is exercising supreme legal caution.
In the coming days there will be more tests on the flaperon and it's expected that these will prove the piece's origin. After that, it will probably be many months before deeper analysis allows any tentative deductions about how the plane may have come down.
The debris is being examined at an aeronautical test centre near Toulouse.
There is little doubt now that the flaperon will be declared the first evidence of MH370 in the coming days as no other Boeing 777s are thought to have crashed in the region.
Relatives of the victims were told about the initial results shortly before Mr Najib's news conference on Wednesday.
Jacquita Gonzales, the wife of crew member Patrick Gomes, said: "Now that they have confirmed it as MH370, I know my husband is no longer of this world but they just can't leave it with this one flaperon."
"We urge them to continue searching until they find the plane and bring it back," she told reporters. "It's not over yet."
She said she still hoped to get her husband's body back so that the family can give him a proper burial and say goodbye.
Malaysia Airlines described confirmation of the flaperon as "a major breakthrough".
In a statement, it said it hoped further evidence would be found in the coming days that would "help resolve this mystery". (BBC)