Investigators into the Germanwings crash which was deliberately caused by a co-pilot have recommended new worldwide rules compelling doctors to tell authorities if a patient could be dangerous.

150 people died when co-pilot Andreas Lubitz flew flight 9525 into a mountain-side in the French Alps after locking the pilot out of the cockpit.

The French investigation team, which published a report into the crash on Sunday, revealed that a doctor recommended Lubitz should be treated in a psychiatric hospital two weeks before the disaster.

Lubitz had begun to show symptoms consistent with a psychotic depressive episode in December 2014 and consulted several doctors over the following months, none of whom alerted aviation authorities or his employer, France's aviation security agency BEA revealed.

Bereaved families have been told Lubitz saw 41 doctors for his condition in recent years.

Under German confidentiality laws, none of the doctors was able to alert his employers to his state of mind.

Philip Bramley, the father of 28-year-old Briton Paul Bramley, who died in the crash, said: "Lubitz should never have been allowed to fly, his mental state had been flagged up more than once."

But he added: "The airline failed in duty of care to act upon it. Now I feel they're trying to relinquish their responsibilities."

At a news conference on Sunday, French investigator Arnaud Desjardins contrasted doctors' behaviour in Germany with what happens in, for example, Canada.

He said: "Our first recommendation is ... we request clear definition of rules to require care providers to inform the authorities when a specific patient's health is very likely to impact public safety.

"We think it's a global issue - several countries worldwide."

Another recommendation, he said, was that because the existing regime of pilot testing may be "inadequate", regular "special examinations" should be carried out on pilots with a "medical history of mental disorder".

He said some countries' approaches to flying with mental illness contrasted with the approach of authorities such as the UK, where "conditions are extremely strict ... (pilots') depressive disorders can only be of a slight or moderate state".

As a result, he added, a third recommendation was that European regulators more clearly define when pilots can be declared fit to fly while taking anti-depressants and that pilots should be provided for if they have to take time off.

Other recommendations included a suggestion that colleagues should be able to safely and securely provide feedback on pilots' mental states.