A subsea cutting device for a subsea blow-out preventer or wellhead comprises a tubular body having an axial throughbore, and configured to be fitted to a casing string, the cutting device further comprising a shearing mechanism in the form of a radial cutting system rotatably mounted within a pressure-containing housing, and comprising a plurality of movable cutters arranged around the throughbore and configured for deployment during rotation successively inwards. The cutting device is useful in a subsea shut off device which is latched to a template at surface and run to the sea floor on a casing string which is drilled into place and converted into a riser on a single trip. For the event when emergency disconnection is required, the device may be used to cut the casing and seal in the well. The system then allows a conventional subsea reconnection of the riser.