The pilot of the plane which crashed into the English Channel with Argentine football player Emiliano Sala on board was not qualified to fly at night, the BBC reported on Saturday.
Sala, 28, died when the plane carrying him from Nantes to Cardiff crashed on January 21.
Regulatory authorities said that David Ibbotson was thought to have been colour-blind, which prevents pilots from flying at night.
They have confirmed that Ibbotson, from Crowle, North Lincolnshire, did not hold a "night rating" on his UK private pilot's licence.
"Colour-blindness stops a pilot from obtaining a night rating straight away, because being able to differentiate between green and red lights is key to flying in the dark," an aviation source told the BBC.
"Flying outside the restrictions of your licence is illegal and that's likely to affect the insurance cover for the flight."
The UK Civil Aviation Authority (CAA) said it would not comment until the Air Accidents Investigation Branch (AAIB) investigation was complete.
The AAIB said licensing "continues to be a focus" of its investigations.
European aviation rules define night as "the time from half an hour after sunset until half an hour before sunrise".
Flight plans indicate the flight scheduled to take the Argentine player for his first training session with Cardiff City had been due to leave Nantes airport at 9 a.m. on January 21.
But the flight was postponed until 7 p.m., at Sala's request.
While Sala's body was recovered from the wreckage of the plane in February, Ibbotson's body has not been found.
--IANS
ksk/bg
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