The workman fell down inside one of the bridge's towers

The man was working on one of the bridge supports on the Welsh side of the estuary when he fell 20ft down inside the structure.

He is believed to have suffered serious head, leg and back injuries in the fall at around 1100 BST on Thursday.

Paramedics and fire crews have been called into rescue the man, and police and coastguards are at the scene as a precaution.

One lane of the westbound carriageway and the hard shoulder of the bridge have been closed while police investigate the incident.

Adrian Mantle of Avon Fire Brigade said: "The fire service and ambulance had to take kit in and walk to him for about 15 minutes because the man was inside the structure of the bridge.

"He was carried to a different access point where I believe he was taken away by an ambulance.

"The whole operation took about three hours."