A series of weekend closures on the M48 Severn Bridge has begun due to work to combat corrosion in suspension cables.
The Highways Agency is planning to install a dehumidification system as part of repairs costing around £20m.
The westbound carriageway is set to be closed on weekends up to 1 July, then the eastbound carriageway will shut on weekends up to 22 July.
Drivers will be diverted to the M4 Second Severn Crossing. Cyclists and pedestrians will not be affected.
Chris Pope, Highways Agency project manager, said safety was the firm's priority at the bridge, which was opened in 1966.
"We can reassure the public that the M48 Severn Bridge continues to operate safely," he said.
"A programme of cable inspections was carried out on the bridge between April 2006 and January this year after corrosion was found in the cables of similar bridges in other parts of the world."
Second phase
The work will see cables wrapped in an impermeable material, while dry air will be blown through the small spaces between the wires inside the cables.
This drying-out is designed to "significantly reduce" the rate of deterioration of the cables, the agency has said, adding that the work can only be conducted during daylight hours and in reasonable weather conditions.
Mr Pope added that a second phase of work to install the dehumidification system to the entire length of the cables was planned for next summer.
When it was opened 41 years ago, the Severn Bridge replaced the ferry service crossing from Aust to Beachley.
The new Second Severn Crossing, around 5km downstream, was opened on 5 June 1996.
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