Air accident investigators say the plane carrying football player Emiliano Sala was not licensed for commercial use by authorities.
It comes as new images were released of the seabed wreckage.
The initial report from the Air Accident Investigation Branch (AAIB) says investigators have yet to determine if it could be classified as a private flight if costs were being shared, so commercial flight regulations were not applicable.
Still from handout video issued by the Air Accidents Investigation Branch Source: AAIB
The United States-registered single-engine aircraft crashed into the English Channel while flying last month from the French city of Nantes to Wales, where Sala was preparing to start a new career with Premier League club Cardiff.
Sala's body was recovered two weeks after the crash but the other person on board - the pilot David Ibbotson - is still missing.
The AAIB says "it is thought that the pilot's licence and logbook were lost with the aircraft and so the ratings on his licences and their validity, and the extent of his recent flying have not yet been determined."
