The Brynglas Tunnels carry the M4 under Brynglas Hill in Newport, Wales. The 360-metre-long (390 yd) tunnels are the first and only twin–bored tunnels in the UK motorway network.

The M4, originally the London-South Wales Motorway, is a motorway in the United Kingdom running from west London to southwest Wales. The English section to the Severn Bridge was constructed between 1961 and 1971; the Welsh element was largely complete by 1980, though a non-motorway section around Briton Ferry bridge remained until 1993. On the opening of the Second Severn Crossing in 1996, the M4 was rerouted over it.