town in Spelthorne borough, administrative county of Surrey, historic county of Middlesex, England, on the left bank of the River Thames. A residential community on the western fringe of Greater London, Staines marks a crossing of the Thames (Thames, River) used since Roman times by the major road from London to the southwest of Britain. Nearby are large reservoirs where Thames water is pumped, purified, and stored for London's Metropolitan Water Board. Pop. (2001) 50,538.