The pilot of the plane that crashed with Cardiff City striker Emiliano Sala on board did not have a licence for commercial flights, investigators say.
The Air Accidents Investigation Branch (AAIB) confirmed David Ibbotson held a private pilot's licence.
Its interim report said he could only fly passengers in the EU on a cost sharing basis, not for reward.
Cardiff City said it had "grave concerns" and questions over the validity of the pilot's licence.
Investigators have not yet been able to establish what arrangement Mr Ibbotson had with Sala, and the status of 59-year-old's licence at the time of the crash on 21 January will form part of the AAIB's investigations.
As flying is an expensive pastime, cost sharing allows pilots to notch up their flying hours while sharing the cost of fuel, landing fees and other expenses with passengers.
Sala was found in the wreckage of the Piper Malibu N264DB, which was found on the seabed 13 days after it vanished over the English Channel near Guernsey.
He had completed his transfer to Premier League side Cardiff from French club Nantes just two days earlier - for a club record fee of £15m - and had returned to France to say goodbye to his former teammates.
Mr Ibbotson has not yet been found and his family hope a fresh search for his body will begin this week after setting up an online fundraising campaign which has raised £250,000.
The aircraft remains underwater off the coast of Guernsey after an attempt to recover it was hampered by bad weather.