Aviation experts have given some credence to claims that one of the pilots in the French Alps plane disaster was locked out of the cockpit before the crash.

The New York Times quoted a senior military source involved in the investigation into the Airbus A320 crash as saying black box evidence reveals a pilot can be heard leaving the cockpit before trying to get back in.

The source said: "The guy outside is knocking lightly on the door and there is no answer. And then he hits the door stronger and no answer. There is never an answer. You can hear he is trying to smash the door down.

"We don't know yet the reason why one of the guys went out. But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door."

David Kaminski-Morrow, air transport editor of the Flightglobal publication, said there had been crashes in Mozambique and Japan where something similar had happened.

He said: "Usually the cockpit door locks and to get in you have to have to put in a code, but inside there is a simple switch on the control panel the pilot would turn to lock the door.

"The 2013 plane crash in Mozambique had circumstances where the pilot left the cockpit and couldn't get back in while the guy in the cockpit took the plane into a nosedive."

He went on: "It looks like this incident has precedent at least consistent with that being a possibility. I'm starting to count the number of fatalities that can be attributed to the cockpit doors and whether its locks are saving lives."

Former Lufthansa engineering technical instructor Stephen Wright, who trains undergraduate pilots at the University of Leeds, said there was an incident 12 months ago in the US where one of the pilots was locked out of the cockpit.

On that occasion the other pilot requested a priority landing and the plane touched down safely.

"The whole case of the Airbus Alps crash is most unusual, " said Dr Wright.

Locked cockpit doors were introduced worldwide after the 9/11 attacks in the US.

The Sydney Morning Herald quoted an unnamed Australian Airbus A320 pilot who said that if the pilot flying the aircraft does not want the other pilot to enter the flight deck, the one in the cockpit can block entry if he reacts before the door opens automatically.

The A320 pilot said: "If the person on the other side of the door says 'no', you can't get in."

He added that the doors are bolted and heavily protected and it would probably be impossible to break one down quickly.

All that investigators of the Germanwings crash have said so far is that the black box cockpit voice recorder contains "usable" data.

A spokesman for the French air investigation bureau, the BEA, said the second black box, the flight data recorder, has yet to be found despite reports it had been recovered and was too badly damaged to be useful.

The investigation is likely to centre on why there was no distress call from the plane, which went into a slow descent even though it was in a mountainous area, and theories include the complete incapacity of the cockpit crew, possibly after a windscreen blow-out.

Earlier, Germanwings' parent company Lufthansa said the plane, piloted by two experienced captains, was "technically flawless", while the firm's chief executive Carsten Spohr, himself a pilot, described the crash as "inexplicable".

Lufthansa said: "We cannot comprehend how a technically flawless airplane steered by two experienced pilots could encounter such a situation at cruising altitude.

"All of us at Lufthansa are working to ensure that such an incident will never occur again. We cannot believe that this has happened. We are doing everything to support the families."

Meanwhile, t he families of three Britons killed in the French Alps disaster have spoken of their loss.

Those who died included seven-month-old Julian Pracz-Bandres, from Manchester, who was killed with his mother Marina Bandres Lopez Belio, 37, originally from Spain.

Another of the Britons to die was senior quality manager Martyn Matthews, 50, from Wolverhampton, who worked at Tipton in the West Midlands.

A third Briton on the Barcelona to Dusseldorf flight was Paul Bramley, 28, who was originally from Hull and was studying hotel management in Lucerne, Switzerland.

Announcing that three Britons had died, Foreign Secretary Philip Hammond said: "We cannot rule out the possibility that there are further British people involved.

"The level of information on the flight manifest doesn't allow us to rule out that possibility until we've completed some further checks."