A doctor referred Germanwings co-pilot Andreas Lubitz to a psychiatric clinic two weeks before he crashed a plane into the French Alps, killing 150 people, French air accident investigators have revealed.

The BEA investigation agency, releasing a report on Sunday on the March 2015 crash, said multiple doctors who treated Lubitz in the weeks before the crash did not inform authorities of concerns about his mental health.

Because Lubitz did not inform anyone of his doctors' warnings, the BEA said "no action could have been taken by the authorities or his employer to prevent him from flying".

Investigators found that Lubitz intentionally crashed Flight 9525 en route from Barcelona to Duesseldorf.

The BEA investigation is separate from a manslaughter investigation by French prosecutors seeking to determine eventual criminal responsibility for the crash.

Investigators are recommending that world aviation bodies define new rules to require that medical professionals warn authorities when a pilot's mental health could threaten public safety.

They made several recommendations to avoid such accidents in the future, notably about pilot mental health and screening before a pilot is certified.

Lubitz had been treated for depression in the past, and the investigation found that he had consulted dozens of doctors in the weeks before the crash.

The pilot certification process failed to identify the risks presented by Lubitz, the report said.

Arnaud Desjardin, leading the investigation, told reporters that experts found Lubitz's symptoms at that time "could be compatible with a psychotic episode".

He said this information "was not delivered to Germanwings".

Investigators said one factor leading to the crash might have been a "lack of clear guidelines in German regulations on when a threat to public safety outweighs" patient privacy.

Mr Desjardin described Germany's privacy rules as being especially strict. He says doctors fear losing their jobs if they unnecessarily report a problem to authorities.

He added "that's why I think clearer rules are needed to preserve public security".

The BEA report also recommends measures to remove the fear of losing a job that many pilots face for mental health issues. It said "the reluctance of pilots to declare their problems and seek medical assistance ... needs to be addressed".

But Mr Desjardin said investigators determined that systematic, deep psychological tests every year for all pilots would be "neither effective nor beneficial".

The report also recommended that airplane cockpit rules should not be changed despite Lubitz locking his pilot out of the control room.

It said that it is still just as important to protect the cockpit from attackers elsewhere in the plane. Current cockpits are equipped with a code system to prevent the kind of hijackings that occurred on September 11, 2001, in the United States.

Mr Desjardin said "a lockage system cannot be created to prevent threats coming from outside and inside the cockpit".

Many airlines and regulators have issued changes since the Germanwings crash and now require at least two people to be in the cockpit at any given time to prevent similar crashes.