An auxiliary trip unit for a circuit breaker comprises: a drive part with a movable blade, a latch fitted pivoting around a swivelling axis and designed to secure the blade in a neutral position, against a flexible bias force, until movement of the latch takes place to a released position, a nose of the latch salient in a direction passing through the swivelling axis so as to comprise a salient end where positive latching of the blade on the latch takes place in the neutral position. The latch collaborates with the blade to receive a thrust force acting in the direction of resetting of the latch to its latching position.