A cutting tool for cutting a casing of a wellbore, 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 (3, 4); a piston mounted within the main body and longitudinally movable with respect to the main body, an upper end of the piston being open, a recess being formed at the opening; 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 19) 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 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, wherein: the flow regulator comprises a blocking portion, which is provided at its upper end, and completely or substantially fills the internal diameter of the main body; and a delivery portion of the flow regulator has a sealing region, which fits closely within the recess, or within an insert in the recess, characterised in that: the delivery portion has one or more flow apertures formed therethrough; a sleeve element is positioned within the delivery portion; in an initial configuration of the tool, the sleeve element is aligned with the flow apertures so that the flow apertures are at least partially occluded by the sleeve element; in a second configuration the flow apertures are exposed, allowing fluid to flow along the second path; and in the second configuration, fluid may flow from the interior of the flow regulator outwardly through the flow apertures. 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 (3, 4); a piston mounted within the main body and longitudinally movable with respect to the main body, an upper end of the piston being open, a recess being formed at the opening; 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 19) 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 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, wherein: the flow regulator comprises a blocking portion, which is provided at its upper end, and completely or substantially fills the internal diameter of the main body; and a delivery portion of the flow regulator has a sealing region, which fits closely within the recess, or within an insert in the recess, characterised in that: the delivery portion has one or more flow apertures formed therethrough; a sleeve element is positioned within the delivery portion; in an initial configuration of the tool, the sleeve element is aligned with the flow apertures so that the flow apertures are at least partially occluded by the sleeve element; in a second configuration the flow apertures are exposed, allowing fluid to flow along the second path; and in the second configuration, fluid may flow from the interior of the flow regulator outwardly through the flow apertures. the flow regulator comprises a blocking portion, which is provided at its upper end, and completely or substantially fills the internal diameter of the main body; and a delivery portion of the flow regulator has a sealing region, which fits closely within the recess, or within an insert in the recess, characterised in that: the delivery portion has one or more flow apertures formed therethrough; a sleeve element is positioned within the delivery portion; in an initial configuration of the tool, the sleeve element is aligned with the flow apertures so that the flow apertures are at least partially occluded by the sleeve element; in a second configuration the flow apertures are exposed, allowing fluid to flow along the second path; and in the second configuration, fluid may flow from the interior of the flow regulator outwardly through the flow apertures. the delivery portion has one or more flow apertures formed therethrough; a sleeve element is positioned within the delivery portion; in an initial configuration of the tool, the sleeve element is aligned with the flow apertures so that the flow apertures are at least partially occluded by the sleeve element; in a second configuration the flow apertures are exposed, allowing fluid to flow along the second path; and in the second configuration, fluid may flow from the interior of the flow regulator outwardly through the flow apertures.