French prosecutors have opened an investigation into the possible manslaughter of 66 people aboard EgyptAir flight MS804 saying there is no evidence the aircraft was downed by a act of terrorism.

Authorities said they were opening the probe before analysis began on the plane's black box. However, just prior to the announcement France's accident investigation agency laboratory was said to have successfully repaired the flight's data recorder, one of two components that make up the black box.

Agnes Thibault-Lecuivre, a spokeswoman for the French prosecutor's office, said French authorities are "not at all" favouring the theory that the plane was downed deliberately, though the status of the inquiry could eventually change if evidence emerges to that effect, the Associated Press reported.

Egyptian authorities, which are leading the investigation and have most closely clung to the theory that the doomed EgyptAir flight was brought down by a terror attack, have said they are unaware of the French investigation. The Middle Eastern country's civil aviation authority has said all possible reasons behind the crash need to remain on the table.

All 66 people, including one Briton, on board the Airbus 320 were killed when it crashed in the Mediterranean early on 19 May while en route from Paris to Cairo.

France's aviation safety agency said automated electronic messages sent by the plane indicated that smoke detectors went off in a toilet and a fault in a flight control unit suggest the plane was lost.

According to information provided by Greek officials, the plane turned 90 degrees left and then 360 degrees to the right before falling into the sea. However, Egyptian authorities have subsequently cast doubt on the claims.

France is able to investigate the circumstances of the crash because the plane was manufactured by France-based Airbus and French citizens were among those killed.

BEA spokesman Sebastien Barthe has said before the black box can be sent to Egypt, technicians at France's air accident investigation agency, the BEA, will attempt to clean and repair it.