Weight restrictions for heavy-goods vehicles will be introduced on the M48 Severn crossing in a bid to future-proof the bridge.
National Highways said only heavy-goods vehicles up to 7.5 tonnes in weight will be able to use the M48 Severn Bridge from 27 May, with those over the threshold directed towards the M4 Prince of Wales bridge instead.
The weight restriction, which will remain in place for 12-18 months, comes after safety inspections found the bridge's main cables are deteriorating.
Chris Pope, programme delivery manager for National Highways, said: "While the bridge remains safe, it was not designed and built for today's levels or weight of traffic."
He added: "Vehicles have got heavier and traffic levels have increased significantly over the past 60 years, putting greater load on the cables.
"Continued use of the bridge by heavier heavy vehicles could ultimately see it closed to all traffic. We are acting now to prevent this from happening."
