British investigators said on Wednesday it was "likely" that two seat cushions that washed up on the French shore this week were from the missing plane carrying footballer Emiliano Sala.

Britain's Air Accidents Investigation Branch (AAIB), which is probing the disappearance of the light aircraft over the Channel on January 21, said the cushions were discovered on the Normandy coastline on Monday.

"From a preliminary examination we have concluded that it is likely that the cushions are from the missing aircraft," it said in a statement.

Sala was flying in the Piper PA-46 Malibu plane after transferring from French team Nantes to Premier League club Cardiff City in a 17-million-euro ($19.3-million) move.

It vanished from radar around 20 kilometres (12 miles) north of the Channel island of Guernsey, with pilot Dave Ibbotson the only other person aboard.

Rescue workers suspended the official search for the aircraft last Thursday after an intensive hunt involving planes and boats failed to locate it.

The AAIB said that with the help of Britain's Ministry of Defence it had now commissioned a "specialist survey vessel" equipped with sonar equipment to scour the seabed for the plane.

"If the wreckage is found, a remotely operated vehicle will be used to visually examine the wreckage," it added.

The agency said due to weather and sea conditions, it expected to start the three-day underwater search at the end of the weekend.

The AAIB said it had identified a priority search area of approximately four square nautical miles "based on a detailed assessment of the flight path and last known radar position".

It was in contact with David Mearns, a shipwreck hunter hired by Sala's family, who announced on Monday his Bluewater Recoveries firm was planning to begin an underwater search on Sunday.

"We are liaising closely with those involved to maximise the chance of locating any wreckage and ensure a safe search operation," the AAIB said.

Mearns said on Monday his survey would focus on an area of roughly 25 square nautical miles where the plane disappeared from radar, where the depth of the sea was between 65 metres (2015 feet) and 110 metres.

"We cannot guarantee that we will find the plane," he told a press conference in Guernsey airport that was shown by British broadcasters.

"The family are determined to get answers that they don't have right now," he added.

Sala's distraught relatives turned to Mearns after their calls for the search to continue went in vain.

The family's appeals were echoed last week by Argentine footballing legend Lionel Messi and the country's President Mauricio Macri.

Meanwhile top players were among the more than 4,500 contributors to a crowdfunding page which has raised over 370,000 euros ($422,000) to pay for the private hunt for 28-year-old Sala and Ibbotson, 59.

On Monday the player's mother and sister, who arrived in Britain last week, flew over the area where the plane disappeared.

Mearns said they now had "a better appreciation" of the efforts made by rescuers from Britain, France and Guernsey who carried out the search last week.