France's civil aviation accident bureau, the BEA, said on Friday that information contained in the flight recorders indicated a fire had broken out in the Airbus A320's cockpit, leading to loss of control of the plane, which crashed in the south-eastern Mediterranean.
Forty Egyptians and 15 French nationals were on board.
The BEA said the crew could be heard discussing a fire on the cockpit voice recorder and that the plane's automatic ACARS messaging system had flagged up smoke on board.
French investigators have always leaned towards a mechanical fault as the cause of the crash, saying they suspected that a mobile phone or tablet had caught fire.
But an official Egyptian report blamed an explosion, saying traces of explosives had been found on the victims' remains.
The BEA said it hoped that its hypothesis would be investigated in the interest of aircraft safety and called on its Egyptian counterpart to publish its report.