British investigators said on Wednesday that 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 to Cardiff.

"From a preliminary examination we have concluded that it is likely that the cushions are from the missing aircraft," said Britain's Air Accidents Investigation Branch.

A single-engined Piper PA-46 Malibu aircraft in which Sala was travelling disappeared over the English Channel on January 19.

Argentine striker Sala had just signed for Cardiff from French club Nantes for a reported fee of €17 million.

The plane disappeared some 20 kilometres north of Guernsey.

"I'm on a plane that looks like it's going to fall apart, and I'm leaving for Cardiff," Sala said in a WhatsApp audio message shortly before the plane disappeared. 

"If in an hour and a half you have no news from me, I don't know if they will send people to look for me, because they will not find me, you know. Dad, I'm so scared," he added.

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 search 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 now 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 said 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.