A cutting tool, comprising: an elongate main body having an inlet end and an outlet end, a fluid flow path being defined between the inlet and the outlet ends; a piston mounted within the main body and longitudinally movable with respect to the main body; one or more cutters, each cutter being moveable between a retracted position and a deployed position, wherein the piston and each cutter engage one another so that longitudinal movement of the piston with respect to the main body moves each cutter between the deployed position and the retracted position; and a flow regulator, operable to divert fluid flowing into the inlet end of the tool selectively along a first path, which passes the through the piston to the outlet end of the tool, and a second path, in which the fluid tends to drive the piston longitudinally with respect to the main body.