A doctor consulted by the German pilot who crashed a Germanwings jet into the Alps last year, killing 150 people, recommended two weeks before the disaster that he should be treated in a psychiatric hospital, French investigators said on Sunday.

The unidentified private physician was one of a number of doctors consulted by Andreas Lubitz as he wrestled with symptoms of a "psychotic depressive episode" that started in December 2014 and may have lasted until the day of the crash, they said.

Prosecutors believe the 27-year-old co-pilot, who had a history of depression, barricaded himself into the cockpit and deliberately propelled his Airbus jet into a mountainside on March 24, 2015, killing everyone on board.

France's BEA air accident investigation office said in its final report that neither Lubitz nor any of his doctors had alerted aviation authorities or his employer about his illness.

It urged the World Health Organization and European Commission to draw up clear rules to oblige doctors to inform authorities when a patient's health is likely to impact public safety, if necessary against the patient's consent.

It also called for tougher inspections when pilots with a history of psychiatric problems are declared fit to fly.

None of the doctors who treated Lubitz, who had been prescribed anti-depressants and sleeping aids, agreed to speak to French or German crash investigators, the agency said.

"In Germany and in France, doctors are very attached to this notion of medical secrecy, but I hope there will be some moves there," BEA Director Remi Jouty told a news conference.

At the same time, the BEA called on airlines to find ways to ease the economic risks for pilots who may fear losing their job if they declare illness and to promote peer support schemes.

Prosecutors have found evidence that Lubitz, who also had eyesight problems and may have feared losing his job, had researched suicide methods and concealed his illness from his employer, sparking a debate on supervision and medical secrecy.

The BEA said his mental state had not generated any concerns reported by the pilots who flew with him.

However it cited a "lack of clear guidelines in German regulations" on when a threat to public safety outweighs the requirements of medical confidentiality.

Lubitz had been flying on a medical certificate that contained a waiver because of a severe depressive episode from August 2008 to July 2009. The waiver stated that the certificate would become invalid if there was a relapse into depression.

The BEA did not recommend any changes to the construction of cockpit doors, which were strengthened after the Sept 11, 2001, attacks in the United States.

Cockpit recordings show that Lubitz disabled the cockpit door-opening mechanism when the captain of flight 4U9525 took a toilet break 30 minutes into the Barcelona-Dussseldorf trip.

Seconds later, he placed the jet on its fatal descent and ignored repeated hammering on the door, cockpit alarms and radio calls from the ground as the airplane sped lower.

Jouty said the cockpit doors had been designed to ward off possible terrorism threats from the passenger cabin and that such risks still existed. It was not possible to design a door lock that addressed risks from both passengers and crew.