British military investigators are to carry out an underwater search for the plane that vanished with the Argentinian footballer Emiliano Sala on board after his country’s government asked the UK to extend the hunt.

The development came as it emerged that two cushions believed to have come from the missing plane have washed up on the coast of France, about 20 miles from the last known location of the aircraft.

Ministry of Defence salvage experts are to work alongside shipwreck specialists hired privately by Sala’s family to search for the plane, which vanished above the Channel Islands on Monday last week en route from France to Cardiff.

The UK’s Air Accidents Investigation Branch (AAIB) has identified a “priority search area” of about four sq nautical miles (14 sq km) where it believes the wreckage of the plane may lie. It has commissioned an MoD salvage and marine operations team to try to locate it. The start of this operation is being delayed by poor weather.

The AAIB said it had been investigating the possibility of searching the seabed since the plane vanished.

The active search for the Cardiff City striker Sala and the pilot, David Ibbotson, was halted last Thursday by the Guernsey harbourmaster, causing an outcry from Sala’s family and a series of Argentinian footballers including Lionel Messi.

On Wednesday evening the Argentinian embassy in London suggested the search had been ramped up again after its intervention.

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In a statement it said: “UK Foreign Affairs Secretary Jeremy Hunt confirmed today that the British government has agreed to the Argentinian government’s request to extend the search for the missing aircraft.

“In a letter addressed to his Argentine counterpart, Jorge Faurie, Jeremy Hunt informed that as soon as the weather conditions allow, the AAIB (Air Accidents Investigation Branch) will start the underwater search. He also expressed his solidarity with Emiliano’s family.”

Earlier on Wednesday it emerged that two blue seat cushions believed to be from the plane had washed up on a French beach.

The AAIB said: “On the morning of Monday 28 January, we were advised by the Bureau d’Enquêtes et d’Analyses, the French safety investigation authority, that part of a seat cushion had been found on a beach near Surtainville on the Cotentin peninsula.

“A second cushion was found in the same area later that day. From a preliminary examination, we have concluded that it is likely that the cushions are from the missing aircraft.”

It added: “Since we opened our safety investigation, we have been gathering evidence such as flight, aircraft and personnel records, and have been analysing radar data and air traffic tapes.

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“From the moment we were notified of the missing aircraft, we have been looking at the feasibility of conducting an underwater seabed search for aircraft wreckage. Based on a detailed assessment of the flight path and last known radar position, we have now identified a priority search area of approximately four square nautical miles. Through the Ministry of Defence’s salvage and marine operations project team, we have commissioned a specialist survey vessel to carry out an underwater survey of the seabed to try to locate and identify possible aircraft wreckage.

“Due to the weather and sea conditions, we currently expect our underwater seabed search to start at the end of this weekend and to take up to three days. Side-scan sonar equipment will be used to try to locate the wreckage on the seabed. If the wreckage is found, a remotely operated vehicle will be used to visually examine the wreckage.”

Shipwreck experts hired by Sala’s family with the help of hundreds of thousands of pounds of donations have been preparing to launch their own search.

This week they revealed they were planning to use a remotely operated vehicle to comb an area of about 25 sq nautical miles and they believed there was a good chance of finding the plane. Both Sala’s experts and the AAIB have said they will be able to work together.