Air searches for the missing Malaysia Airlines plane are being cancelled as authorities begin a new underwater operation that could take up to eight months.

Tony Abbott, the Australian prime minister, said it was "highly unlikely" that wreckage from the missing Boeing 777 will ever be found on the ocean surface, as the material would have sunk.

The new phase of the search, which has been ongoing since 8 March, will focus on submerged operations using sophisticated sensors. The search was originally narrowed to where officials believed the plane's black boxes were located, but the area will now be broadened to an area 430 miles long and 50 miles wide, where the aircraft is though to have ended its flight.

Abbott said the intensified submerged search was necessary after the "disappointing and baffling" failure to find the wreckage. He added that it could take between six to eight months.

The new search will need to cover more than 20,000 square miles. Australia has agreed to pay the £33 million cost but will consult with other nations.

"I am now required to say to you that it is highly unlikely at this stage that we will find any aircraft debris on the ocean surface," Abbott said. "By this stage, 52 days into the search, most material would have become waterlogged and sunk."

He said that the US autonomous mini-submarine, Bluefin-21, had completed its intial search off Australia's western coast without finding any debris.

Abbott added that the search for MH370 will continue, despite being unsuccessful so far.

"I want the families to know, I want the world to know, that Australia will not shirk its responsibilities in the area. We will do everything we humanly can... to solve this mystery. We will not let people down and while the search will be moving to a new phase in coming weeks, it certainly is not ending," he said.

Yet he acknowledged that it was possibly that no material from the aircraft would ever be found, saying: "Of course it's possible, but that would be a terrible outcome because it would leave families with a baffling uncertainty forever. The aircraft plainly cannot disappear - it must be somewhere."

The plane, which was carrying 239 passengers and crew, disappeared on 8 March. Satellite and radar data show the aircraft diverted away from its original route mid-way through a flight from Kuala Lumpur to Beijing. Officials believe the plane ended its flight in a remote area of the southern Indian Ocean.

Angus Houston, air chief marshall who is leading the search, added: "If everything goes perfectly, I would say we'll be doing well if we do it in eight months."

But he said that weather conditions and technical issues with equipment could extend the operation.

The Malaysian government is expected to release a preliminary report on the plane's disappearance this week.