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 experts examining the debris in France had "conclusively confirmed" it was from the aircraft.
But the investigators have stopped short of confirming the link, saying only that it is highly likely.
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 and was taken to Toulouse for testing.
The plane is long believed to have crashed into the southern Indian - 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 in Kuala Lumpur.
"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".
'Very strong indications'
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.
Analysis: Hugh Schofield, BBC News, Paris
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.
'It's not over yet'
Australia has been leading the search for the plane in the area it is believed to have gone down, some 4,000km (2,500 miles) east of Reunion.
Prime Minister Tony Abbott told Melbourne Radio 3AW the discovery "does seem to confirm that it went down in the Indian Ocean, it does seem very consistent with the search pattern that we've been using for the last few months".
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".