Malaysian Prime Minister Najib Razak told a press conference in Kuala Lumpur late last night that new data showed the plane, carrying 239 people, had crashed into the ocean west of Perth after going missing more than two weeks ago.
"Based on new analysis we have concluded [the jet] flew along southern corridor and that its last position was in the middle of Indian Ocean west of Perth.
"This is a remote location far from any possible landing site.
"It is therefore with deep sadness and regret that I must inform that in accordance with this new data Flight MH370 ended in the southern Indian Ocean."
Mr Najib said he had been briefed by representatives from the UK Air Accident Investigation Branch (AAIB), which informed him that satellite data from the UK company Inmarsat, using techniques "never before used in an investigation of this sort", revealed the final position of the plane.
Read more of the Herald's Flight 370 coverage today:
• Raw grief as relatives hear the news
• Hopes high amid gruelling search
• MH370: What we know now
The International search effort had been expected today to zero in on an area where two objects had been sighted by an Australian crew the day before.
Prime Minister Tony Abbott told parliament yesterday evening that a Royal Australian Air Force P3 Orion had located two objects, a grey or green circular object and an orange rectangular object.
The HMAS Success is in the area, about 2500km southwest of Perth, attempting to recover the objects.
However due to the weather it has now been diverted to an area south of the search zone until conditions improve.
A US Navy P8 Poseidon, a second RAAF Orion and a Japanese Orion had also been en route to try to find the items.
Malaysian authorities last night the objects could be retrieved "by (Tuesday) morning at the latest''.
However that's now unlikely.
The most recent sightings have followed suspected plane debris being picked up by Australian, Chinese and French satellites.
Flight 370 vanished on March 8 just an hour into its journey from Kuala Lumpur to Beijing, carrying 239 people, including six Australians and two New Zealanders.
How relatives were told
Relatives of passengers and crew have been told of the "heartbreaking'' news that flight went down in southern Indian Ocean.
Moments before the Prime Minister's press conference, officials from Malaysia Airlines sent an SMS text message to the families of the passengers on board when the plane was lost which said: "We must now accept all evidence suggests the plane went down in the Southern Indian Ocean".
"Malaysia Airlines deeply regrets that we have to assume beyond any reasonable doubt that MH370 has been lost and that none of those on board survived," the message read.
"Malaysia Airlines has already spoken to the family of the passengers and crew to inform them of this development," Mr Najib said.
"For them the past few weeks have been heartbreaking. I know this news must be hard as well."
Bad weather to hamper search
Earlier, the Malaysia transport minister, Hishammuddin Hussein, said an Australian naval ship could locate possible debris within hours.
Earlier today New Zealand Air Force Air Commodore Mike Yardley had predicted bad weather could significantly hamper those efforts.
The weather system approaching the search area will bring "significantly" bad weather, he told TV3's Firstline this morning.
"We've got a bad front coming through with north-west gales as I understand, so the sea state is going to rise and that means it'll be very difficult conditions today for our visual look-outs and also challenging for our radar," he said.
"I understand in fact that the weather might be so difficult the Australian authorities are now telling us that they don't think [HMAS] Success will now be in a position to collect any of this debris even if we find it."
Video: Chinese, Australian planes spot possible debris
So far, ships in the search effort have been unable to locate several "suspicious" objects spotted by satellites in grainy images or by fast-flying aircraft over a vast search area in the remote southern Indian Ocean.
Earlier yesterday spotters on a Chinese plane said they had seen two white, square-shaped objects in the southern Indian Ocean, at that stage the second possible sighting of plane debris made with the naked eye in the search for the missing Malaysia Airlines Boeing 777.
Spotters aboard that search plane reported the coordinates to a Chinese icebreaker ship, Xue Long, which was making its way to the area - as well as to the central Australian command centre.
In addition to the two larger floating objects, the searchers also reported seeing a range of smaller, white debris scattered over several square miles, according to China's Xinhua news agency.
The sightings were all made in the area identified in previous satellite images from Australia and China.
The developments came as the US prepared to send a specialised device that can locate black boxes into the region.
- additional reporting AAP
