A downhole cutting tool (10,110) comprising: a tool body; a first flow pathway through the tool body; a second flow pathway through the tool body; a cutting mechanism (18,118) configured to be in fluid communication with the second fluid flow pathway and a switching mechanism; the switching mechanism comprising a mandrel (14,114) having a central mandrel bore with a first end configured to be coupled to an upper tool string, and a first set of channels or ports at a second end, the switching mechanism operable to close the first flow pathway; the cutting mechanism (18,118) having a plurality of knives to cut casing, the tool body having a first end surrounding a portion of the mandrel and a second end configured to be coupled to a lower tool string; the downhole cutting tool further comprising a piston (40,140) axially moveable in a chamber (62,162) of the tool body and comprising a piston sleeve with a shoulder configured to engage a pivot arm of the cutting mechanism, a piston inlet nozzle to a central piston bore and second set of ports (54,154) extending into the central piston bore; and a bypass channel (38,138) in fluid communication with ports (54,154) on the piston (40,140); the downhole cutting tool being switchable by operation of the switching mechanism between a first position and a second position, wherein: in the first position: the first flow pathway is open, and fluid flow from the upper tool string enters the central mandrel bore, passes through the first set of channels or ports into the bypass channel (38,138) to enter the second set of ports (54,154) extending into the central piston bore and to an inner bore of the lower tool string, and the knives are retracted and held in a storage position; and in the second position: the second flow pathway is open, the first flow pathway is closed as the bypass channel (38,138) is closed, and fluid flow from the upper tool string enters the central mandrel bore, passes through a third set of ports (34,184) into the chamber (62,162) to enter the inlet nozzle to the central piston bore and moves the piston to engage a second shoulder with the pivot arm to rotate the knives to an extended operational position to cut casing. a tool body; a first flow pathway through the tool body; a second flow pathway through the tool body; a cutting mechanism (18,118) configured to be in fluid communication with the second fluid flow pathway and a switching mechanism; the switching mechanism comprising a mandrel (14,114) having a central mandrel bore with a first end configured to be coupled to an upper tool string, and a first set of channels or ports at a second end, the switching mechanism operable to close the first flow pathway; the cutting mechanism (18,118) having a plurality of knives to cut casing, the tool body having a first end surrounding a portion of the mandrel and a second end configured to be coupled to a lower tool string; the downhole cutting tool further comprising a piston (40,140) axially moveable in a chamber (62,162) of the tool body and comprising a piston sleeve with a shoulder configured to engage a pivot arm of the cutting mechanism, a piston inlet nozzle to a central piston bore and second set of ports (54,154) extending into the central piston bore; and a bypass channel (38,138) in fluid communication with ports (54,154) on the piston (40,140); the downhole cutting tool being switchable by operation of the switching mechanism between a first position and a second position, wherein: in the first position: the first flow pathway is open, and fluid flow from the upper tool string enters the central mandrel bore, passes through the first set of channels or ports into the bypass channel (38,138) to enter the second set of ports (54,154) extending into the central piston bore and to an inner bore of the lower tool string, and the knives are retracted and held in a storage position; and in the second position: the second flow pathway is open, the first flow pathway is closed as the bypass channel (38,138) is closed, and fluid flow from the upper tool string enters the central mandrel bore, passes through a third set of ports (34,184) into the chamber (62,162) to enter the inlet nozzle to the central piston bore and moves the piston to engage a second shoulder with the pivot arm to rotate the knives to an extended operational position to cut casing. the first flow pathway is open, and fluid flow from the upper tool string enters the central mandrel bore, passes through the first set of channels or ports into the bypass channel (38,138) to enter the second set of ports (54,154) extending into the central piston bore and to an inner bore of the lower tool string, and the knives are retracted and held in a storage position; and in the second position: the second flow pathway is open, the first flow pathway is closed as the bypass channel (38,138) is closed, and fluid flow from the upper tool string enters the central mandrel bore, passes through a third set of ports (34,184) into the chamber (62,162) to enter the inlet nozzle to the central piston bore and moves the piston to engage a second shoulder with the pivot arm to rotate the knives to an extended operational position to cut casing.