The invention relates to an actuator for operating a positioning element. A common requirement in such actuators is that the positioning element can be decoupled from the electric motor that drives the positioning element via a transmission. In some applications the actuators are so compact that the only way to decouple the actuator is to disassemble parts of the actuator. To connect the positioning element to the electric motor again, this however requires that the whole actuator is demounted so as to ensure that the disassembled parts are properly assembled. The present invention overcomes this problem by providing an actuator comprising two toothed wheels which by mutual displacement can be brought out of mesh and which each comprises at least two toothed rims positioned with a spacing corresponding to at least the width of one toothed rim. Hence the provided actuator is very compact in that the space required for disengagement of the positioning element is practically reduced to the width of the toothed wheels. Further the construction ensures a firm mesh, making the actuator highly reliable.