|Distance:||0.8 miles (1.3 km)|
|Meets:||A11, B140, A13|
|Route outline (key)|
The B121 is an urban B-road in East London.
It starts on the A11 Mile End Road near Stepney Green tube station and heads southeast to Stepney Green. It TOTSOs right at a mini-roundabout by the maths and computing college - the route straight on is the B140. The road then passes St Dunstan's Church on Belgrave Road.
From there the official line of the road bears left into White Horse Road; however, this turning is closed to traffic and so the obvious route continues along Belgrave Road. Both end on the A13 Commercial Road near Limehouse station. White Horse Road is accessed from traffic lights where the A1203 spur is also met; the road is initially one-way northbound.