The A1203 is located in east London, running between Tower Hill and the Limehouse Tunnel Approach.
The road starts at traffic lights by the Tower of London, where it meets the A100 and A1210 and heads east along Minories, which soon becomes East Smithfield, passing to the north of St Katherine's Dock. The A1202 is met at traffic lights, then the road continues east along The Highway, passing to the north of Wapping and reaching Shadwell. After passing the King Edward VII Memorial Park the A101Rotherhithe Tunnel comes in from the right and runs under, or just to the north of, the A1203, although at this point there is no sign it is here.
We soon reach traffic lights just before the entrance to the Limehouse Link Tunnel. Prohibited traffic is required to turn left here to head along what appears to be a spur of the A1203 north along Butcher Row (the OS claim this is numbered A126 - but that road exists in south Essex) to end on the A13 Commercial Road. This point marks the end of the Rotherhithe Tunnel and there are steps down from Butcher Row to the A101. As such, that road climbs whilst the A1203 descends; our tunnel (technically two parallel tunnels) is modern, however, dating back to 1993.
Even before entry to the Limehouse Link Tunnel traffic is required to get in lane: left for the Isle of Dogs and right for the Royal Docks. The reason for this is that after we have passed under the Limehouse Basin the road forks under Ropemakers Field, with the left fork to the Isle of Dogs coming out of tunnel to reach traffic lights on the A1206 Westferry Road. The mainline bears left to pass under the DLR, with the tunnel opening out in the central reservation of the A1261 in the vicinity of Poplar station. That road number then continues ahead even though it has to TOTSO.
In 1922 the road had the same route west of the tunnel but then turned north up Branch Road to end on the A13 Commercial Road in the same place as the A101 Rotherhithe Tunnel.