French investigators probing the Germanwings plane crash called on Sunday for clearer rules on the lifting of medical confidentiality if pilots show signs of pyschological problems.

The plane was deliberately flown into a French mountainside by its co-pilot Andreas Lubitz in March last year in a tragedy that cost 150 lives and raised unprecedented aviation safety questions.

In their final conclusions on the disaster, BEA civil aviation experts also recommended more stringent medical checks for pilots.

Lubitz, 27, who was suffering from depression, was allowed to continue flying despite having been seen by doctors dozens of times in the years preceding the crash.

READ: Medicine for psychological illness found at co-pilot's home: report

"Clearer rules are needed to establish when it is necessary to lift medical confidentiality," investigator Arnaud Desjardins said at the launch of the BEA report.

"Several doctors in private practice had the information (that Lubitz) was ill," he said.

"This information was not passed on to aeronautical authorities or to his employer Germanwings."

The report also recommended regular analysis of pilots to check for "psychological or psychiatric problems".

In the fateful flight on March 24, 2015, Lubitz locked the pilot out of the cockpit of the Airbus 320 operated by Germanwings, the budget airline subsidiary of Lufthansa.

Ten minutes later, the jet ploughed into an Alpine mountain, killing all 144 passengers and six crew.

Despite being seen so many times by doctors, under German law none was able to alert his employers to his state of mind and he was allowed to continue flying.

- Training interrupted -

On the black box voice recorder recovered at the crash scene, all that is heard from Lubitz is regular breathing. He gave no words of explanation for his murderous course of action.

The European Aviation Safety Agency (EASA) has already recommended stepping up medical testing for pilots, including more psychological tests.

BEA chief Remi Jouty said the French investigation had sought to identify the "systematic failures which led to this accident".

The investigators had also looked at the "balance between medical secrecy and flight security".

As a result of the Germanwings crash, European aviation authorities have already recommended making it compulsory to have two people in the cockpit at any time during flights.

Some countries are opposed to the measure, however, with Germany's pilots' union believing it poses "risks that outweigh any supposed improvements in security".

The dead in the Germanwings disaster included 72 Germans, including a group of 16 high school students, and 50 Spaniards.

A German lawyer for some of the families of the dead said this month they intended to sue the training school in Phoenix, Arizona, which Lubitz attended, claiming it should have flagged up his psychological problems.

"The co-pilot interrupted his training there for a while due to psychological problems," lawyer Christof Wellens said. "He shouldn't have been allowed to resume his training."

Lufthansa has paid 50,000 euros ($56,000) per victim in an initial payment and offered an additional 25,000 euros to each of the families plus 10,000 euros to each immediate relative including parents, children and spouse.