The Diocese of London forms part of the Church of England's Province of Canterbury in England. Historically the diocese covered a large area north of the Thames and bordered the dioceses of Norwich and Lincoln to the north and west. The present diocese covers 177 square miles (460 km2) and 17 London boroughs, covering most of Greater London north of the River Thames and west of the River Lea. This area covers nearly all of the historic county of Middlesex. It includes the City of London in which lies its cathedral, St Paul's, and also encompasses Spelthorne which was formerly in Middlesex but is now part of Surrey.