For the second year running, the main cause of aviation deaths in 2015 was "unlawful interference" -- in other words, murder -- according to a leading air-safety firm.

The Dutch safety consultancy, To70, said: "The total number of fatalities as a result of unlawful interference has increased in the past two years to over 900." It cites the losses last year of two Malaysia Airlines jets -- the still-unsolved disappearance of MH370, and MH17 which was shot down over in Ukraine -- and the two crashes this year involving Germanwings in the Alps and Metrojet in Sinai.

Adrian Young, Senior Aviation Consultant for To70, said: "Unlawful interference on board by passengers is reasonably well covered around the world. My main concerns are centred on the way airport and airline staff get airside. There are many airports that have weak systems to control who goes airside and with what."

The firm's figures assume that the 224 victims aboard the Metrojet flight from Sharm el Sheikh to St Petersburg last October were killed by a terrorist bomb. While Egyptian investigators say they have found no indication of explosives, the Kremlin says it has proof that a bomb planted on board had downed the Airbus A321.

Philip Baum, editor of Aviation Security International, said: "All the indicators are that the explosion which took place was caused by an improvised explosive device."

The UK is working on the assumption that someone planted a bomb on the Russian plane as it waited on the ground at Sharm el Sheikh airport. British airlines are still banned from flying to and from Egypt's main resort because of security concerns.

But Mr Baum said: "The harsh reality is that whichever airport the doomed flight had departed from, there would be similar stories told. Sharm el-Sheikh may well not be an example of best security practice, but it does not stand alone."

The second-highest death toll of the year was 150, aboard Germanwings flight 9525 from Barcelona to Dusseldorf.

Andreas Lubitz, the co-pilot, chose to murder everyone on board while taking his own life. When the Captain briefly left the cockpit, the First Officer locked the flight-deck door then set the controls of the Airbus A320 on a fatal descent into the French Alps.

German police later discovered that Lubitz had a history of severe depression.

The Metrojet and Germanwings tragedies claimed a total of 374 lives. The year's three other commercial aircraft crashes, according to the aviation safety website AirSafe.com, were all domestic flights in Asia: two in Indonesia and one in Taiwan. There were 109 victims in total.

UK airlines enjoyed another fatality-free year. The last time a British passenger jet crashed with loss of life was at Kegworth, 27 years ago this week, in which 47 people died.

The most serious event last year involving a British aircraft took place on the runway at Las Vegas on September 8. The port engine of a BA Boeing 777 caught fire during the take-off roll. All 158 passengers and 13 crew escaped without serious injury.