The B540 is a short link road in north London. The road was originally unclassified but the date of classification is not known, except that it had happened by the 1950s.
The road starts at traffic lights on the B519 in Highgate, opposite the domed St Joseph's Church. This junction is almost exactly at the meeting point of the London Boroughs of Camden, Haringey and Islington and the B540 runs along the boundary of the latter two for its entire length.
We head northeastwards along the traffic-calmed Hornsey Lane, passing St Aloysius College and then crossing the Archway Bridge over the A1. The road continues through the suburbs and presently the road ends at a triangular T-junction on the A103 to the south of Crouch End.