The present invention relates to a power supply system for underwater vehicles, in particular to a power supply system for autonomous underwater vehicles, to underwater vehicles equipped with such power supply systems and to a method of operating an underwater vehicle. The power supply system for underwater vehicles comprises a hydrogen fuel cell, which on the one hand is in fluid contact with a metal hydride storage tank, and on the other hand, with a membrane module that is capable of extracting dissolved oxygen from water. By combining the above mentioned components, the energy necessary to support the AUV operation and the operation of its sensors can be provided, replacing in an efficient and sustainable way the currently employed battery energy systems. For the operation of gliders, a weight compensating mechanism could also be implemented.