Investigators believe co-pilot Andreas Lubitz, 27, locked his captain out of the cockpit when he left to go to the toilet – and then put the Germanwings Airbus A320 into a controlled descent before deliberately slamming it into the side of a mountain. Victims of the disaster included a party of 16 German schoolchildren returning home after an exchange trip to Spain.

Now a host of airlines – including Easyjet, Air Canada, Virgin, Monarch and Thomas Cook – have confirmed that they will be introducing ‘a rule of two’ to ensure that more than one crew member must always be present on the flight deck.

The safety policy – which is already standard practice with many US carriers – is also being introduced by Lufthansa, owner of the  Germanwings budget subsidiary.

Airlines deliberately made cockpits harder to access following the September 11 terror attacks of 2001, when four American passenger jets were hijacked in mid-air with the eventual loss of almost 3000 lives. However, the dreadful fate of flight 9525 this week appears to show that such enhanced security measures can themselves prove lethal with a rogue pilot at the controls.

Flight 9525 took off from Barcelona bound for Dusseldorf on Tuesday morning. The journey proceeded normally until the plane reached its cruising height of 38,000ft, but about a minute later it began its eight-minute descent.

Investigators recovered the black box cockpit recorder from the wreckage and quickly learned that the captain, Patrick Sondheimer, had left the cabin shortly after the plane reached cruising altitude. But he was unable to re-enter, and his increasingly frantic efforts to get back in can be heard as he knocks ever-more urgently before eventually attempting in vain to break the door down. It has been revealed that the harrowing recording also picks up the screams of passengers shortly before the jet disintegrates on ploughing into the mountainside.

Lubitz does not utter a single word, but his breathing is heard on the recording and investigators believe he was alive and conscious until the moment of impact. They think he deployed a five-minute override function to prevent Sondheimer being able to re-enter the cockpit even when he punched in an emergency number to open the door.

It has emerged that Lubitz may have been suffering from depression following a relationship break-up but had hidden his illness from his employers.

A Germanwings statement said: “We are horrified to discover…that the aircraft that crashed in the south of France appears to have been crashed deliberately – probably by the co-pilot of flight 4U9525. Based on audio taken from the voice recorder, the French authorities have come to the conclusion that after the aircraft had reached cruising altitude, the captain left the cockpit for a short time and was then unable to re-enter.

“It appears that the co-pilot, who had stayed in the cockpit, prevented the captain from re-entering by fully locking the cockpit door in order to then initiate the fatal descent. All Germanwings and Lufthansa employees are deeply shocked. We could never have imagined that a tragedy like this could occur within our company.”