Marseille prosecutor Brice Robin, citing information from the "black box" voice recorder, said the co-pilot was alone in the cockpit.

He intentionally started a descent while the pilot was locked out.

Mr Robin said there was "absolute silence in the cockpit" as the pilot fought to re-enter it.

The co-pilot, now named as Andreas Lubitz, was alive until the final impact, the prosecutors added.

"We hear the pilot ask the co-pilot to take control of the plane and we hear at the same time the sound of a seat moving backwards and the sound of a door closing," Mr Robin told reporters.

"At that moment, the co-pilot is controlling the plane by himself. While he is alone, the co-pilot presses the buttons of the flight monitoring system to put into action the descent of the aeroplane.

"This action on the altitude controls can only be deliberate."

The Airbus 320 from Barcelona to Duesseldorf hit a mountain, killing 150 people, after an eight-minute descent.