Malaysia Airlines now believes missing flight MH370 has gone down in the southern Indian Ocean with no survivors.
Malaysian Prime Minister Najib Razak called an impromptu press conference with less than one hour's notice to announce the news.
Mr Razak said analysis with never-before-used techniques by the UK Air Accidents Investigation Branch and Inmarsat, a British-based satellite company, showed the plane was over the southern Indian Ocean when it last transmitted to a satellite.
"This is a remote location, far from any possible landing sites. It is therefore with deep sadness and regret that I must inform you that according to this new data flight MH370 ended in the southern Indian Ocean," he said.
The Prime Minister gave no indication of exactly where in the Indian Ocean the plane was last heard from, or what the next step in finding it would be. The hunt could take years, if the plane is ever found at all.
A text message was sent to family members who were unable to attend a private briefing in Beijing. It read: "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. As you will hear in the next hour from Malaysia's Prime Minister, we must now accept all evidence suggests the plane went down in the Southern Indian Ocean."
Family members leaving the briefing cried uncontrollably, and some collapsed with grief after learning the news.
Mr Razak's announcement marked only the second time the Malaysian PM has spoken out. The first was to announce the plane's movements had been determined to be deliberate.
The press conference was a departure from the normal routine, which saw one held each day.
Yesterday afternoon a Chinese Ilyushin Il-76 spotted what may have been debris from MH370 on its return to Perth. These objects were unable to be located again, but several hours later a Royal Australian Air Force P3 Orion spotted different debris.
Photographs were taken during both sightings, with analysis expected to be made public later today.
Three Chinese vessels and two search and rescue vessels are also due to arrive in the search area today.
US agencies have previously announced they will move a black-box locator into the area once there is an idea where the majority of the debris is located.
The Boeing 777, which took off from Kuala Lumpur for Beijing on March 8, had 239 passengers and crew on board - including two New Zealanders.