BLACK box recordings reveal one of the pilots on the doomed Germanwings flight was locked out of the cockpit before the crash, according to reports.

Investigators said "usable" data had been recovered from the cockpit voice recorder, which they hope may shed light on the lead-up to the mystery crash, but have not given details of any conversations.

But the New York Times quoted a senior military source involved in the probe as saying 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."

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

Those who died in the Germanwings Airbus A320 crash included seven-month-old Julian Pracz-Bandres, from Manchester, who was killed alongside 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."

In the Commons, Prime Minister David Cameron offered his "deepest condolences" to those who had lost loved ones in the crash.

Ms Bandres Lopez Belio's husband Pawel Pracz said she had been visiting her family in Spain for her uncle's funeral and bought the tickets at the last moment.

He said in a statement released by the Foreign Office (FO): "I'm with my closest family in Manchester, and in close contact with our family in Spain at this very difficult time.

"We are devastated and would like to request that we be allowed to grieve in peace as a family without intrusion at this difficult time."

An FO statement on behalf of Mr Bramley's family said: "Paul was originally from Hull. He was studying hospitality and hotel management at Ceasar Ritz College in Lucerne and about to start an internship on April 1.

"Paul had just finished his first year at the college and had taken a few days holiday with friends in Barcelona, before flying back to the UK via Dusseldorf to meet his family."

Mr Bramley's mother Carol lives in Majorca but is currently in the UK, having flown over to meet him.

She said: "Paul was a kind, caring and loving son. He was the best son, he was my world."

Mr Bramley's father Philip, who lives in Hull, said they were both deeply shocked and would miss him.

Ms Bandres Lopez Belio had been living in Manchester for seven years with Mr Pracz, both working in the film and video industry.

Mr Matthews had two grown-up children and worked for German automotive manufacturer Huf. He is thought to have been travelling to Germany for a business meeting.

His family said: "We are devastated at the news of this tragic incident and request that we are allowed to deal with this terrible news without intrusion at this difficult time."

The three Britons were among 150 on the Airbus.

While some nationalities were still to be verified, Germanwings said the dead included 72 Germans, 35 Spaniards and two Australians. Two Americans are also known to have been on the plane.

Other countries with passengers on the flight included Iran, Israel, Japan, Denmark, Belgium and Colombia.