A high or very high voltage circuit breaker comprising, for each phase: at least two breaker chambers connected electrically in series and disposed parallel to each other in a metal tank, in which the ends of the chambers through which at least one pair of movable contacts of each chamber is driven lie in a common plane; and the mechanical transmission assembly whereby simultaneous movement is transmitted, and which ensures identical stroke lengths for the movements of the movable contacts of at least two chambers, comprises firstly at least two bars of identical lengths, each coupled to the pair of movable contacts of one of the chambers, and secondly, drive means coupled to at least one drive rod outside the tank or passing through its wall.