An investigation into the plane crash which killed footballer Emiliano Sala will focus on the validity of the pilot's licence.

The UK's Air Accidents Investigation Branch (AAIB) named "regulatory requirements" as one of four areas in which further work will be carried out.

Its interim report noted that the type of licence held by the pilot meant he could only fly passengers in the European Union on a cost sharing basis, rather than for commercial flights.

Pilots with his licence "must have a bona fide purpose for making the flight", according to the AAIB.

Argentinian footballer Sala had just signed for Cardiff City from French club Nantes for £15 million when the plane crashed into the Channel on 21 January.

He was the only passenger on board the Piper Malibu aircraft flying from Nantes to the Welsh capital.

His body was recovered on 6 February but pilot David Ibbotson, 59, of Crowle, Lincolnshire, has not been located.

Mr Ibbotson's family hope a fresh search for his body will begin this week after setting up an online fundraising campaign which has reached £250,000.

The aircraft remains underwater off the coast of Guernsey after an attempt to recover it was hampered by bad weather. 

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The AAIB report today said the regulatory requirements needed include "airworthiness requirements, aircraft permissions and flight crew licencing".

"Some operational aspects are yet to be determined, such as the validity of the pilot’s licence and ratings.

"Our priority now is to go through the evidence, much of which is extensive and complex, so we can piece together what happened between the aircraft being lost from radar and it coming to rest on the sea bed. This will help us understand the potential causes of the accident.

"We continue to speak to the families of the pilot and passenger to keep them updated on the progress of our investigation. If any urgent safety issues arise during our investigation, we will issue a further Special Bulletin. When our investigation has concluded, we will publish a final report."

There will also be more work to analyse the radar information to try to understand the last few minutes of the flight, and assess "the possible implications of the weather conditions in the area at the time of the accident".

And there will be more analysis of video from the remotely operated underwater vehicle (ROV) "to determine the aircraft attitude as it entered the water".

The remit of the AAIB, which is a branch of the UK's Department for Transport, is to conduct safety investigations into the cause of aviation accidents without apportioning blame or liability.