A magnetic cleaning tool for collecting ferrous debris in a wellbore and a method of cleaning a wellbore. The magnetic cleaning tool has a tool body for attachment in a work string, one or more magnets supported on the tool body, and a sleeve located around the tool body which is arranged to be moveable along the body to provide selective coverage of the magnets. Embodiments are described providing shear pins and spring loaded balls to hold the sleeve either over the magnets or free of the magnets. An expandable piston, moveable by action of a drop ball, is described to release the sleeve. The sleeve is arranged so that the magnets can be covered and therefore effectively inactive on run-in and during downhole operations, and uncovered to be active when pulled out of the wellbore.