Quoting sources close to the French investigating authorities, the German newspaper Bild said that the co-pilot, 27-year-old Andreas Lubitz, appeared to have tried out a "controlled, minute-long descent for which there was no aeronautical reason" on the earlier outbound flight from Düsseldorf to Barcelona.

Investigators had discovered this after a close evaluation of the jet's black-box flight recorder, Bild said.

Investigators in Paris later backed the claim that Lubitz had practiced his decent.

The BEA crash investigations office said Lubitz, had practiced the manouevre on a flight from Düsseldorf to Barcelona
without "noticeable effect" before crashing the plane into the mountains on his return, killing all 150 people on board.

The Germanwings Airbus 320 was en route from Barcelona to Düsseldorf when it crashed in the French Alps on March 24, killing all 150 people on board.

French investigators believe that Lubitz, who had been diagnosed as suicidal in the past, deliberately brought the plane down.

Doctors had recently found no sign that he intended to hurt himself or others, but he was receiving treatment from neurologists and psychiatrists who had signed him off sick from work a number of times, including on the day of the crash.

Police found torn-up sick notes during a search of his apartment after the crash.