On-board security guards could be employed to overpower would-be hijackers before they manage to gain control of the plane.

The US is to expand its air marshal programme and it is believed that El Al already employs undercover guards on its fleet of some 20 aircraft.

The US Federal Aviation Administration is recruiting new volunteers in the wake of the disaster in America though aviation experts warn that they need to be highly trained to be effective.

Opinions are also split about whether their presence would further endanger aircraft - with hijackers detonating bombs if challenged.