A deadly plane crash over the Mediterranean that killed 66 people was caused by a pilot smoking a cigarette in the cockpit, it has emerged. 

Flight MS804 was travelling from France to Egypt in May 2016 when it crashed into the sea south of Crete, killing everyone on board the Airbus A320. Among them were 12 French tourists, a Briton, 30 Egyptians, two Iraqis and a Canadian.

The Egyptian authorities claimed at the time that the plane was brought down by a terrorist attack – despite no group ever claiming responsibility.

But an official investigation has concluded that it was caused by a cigarette being smoked in the cockpit that inadvertently ignited oxygen leaking from an emergency gas mask. 

Egyptian pilots habitually smoked in the cockpit and the practice was not prohibited at the time of the crash in 2016, according to a 134-page report produced by aviation experts that has been obtained by an Italian newspaper, Corriere della Sera.

For reasons that have not been determined, the setting on the oxygen mask had been switched by a maintenance engineer from normal to emergency, causing it to emit oxygen.

Investigators identified a hissing sound – made by the oxygen escaping from the mask – at around 2.25am on May 19, a few minutes before the aircraft crashed into the sea. The report has been sent to the Court of Appeal in Paris. 

The pilots of the plane should have detected the faulty oxygen mask before taking off, an experienced pilot said.

“When we go into the cabin, among the various checks we make before taking off is to check the flow of oxygen in the masks,” Daniele Veronelli, an A320 pilot and a member of Italy’s national association of commercial aviation pilots, told Corriere della Sera.

“If the switch is in the normal position, the flow of oxygen is on request. If it is on the emergency setting, it will release oxygen at a greater pressure to blow away the smoke that could be in the cabin if there’s a fire on board.”