Inspection work is to be carried out on the main suspension cables of the old Severn Bridge.
The bridge celebrates its 40th anniversary in September
The Highways Agency said it was a precautionary measure after cables on the Forth Bridge in Scotland were found to be corroded.
The M48 bridge, which links England to Wales at Aust in South Gloucestershire, will be 40 years old in September.
The detailed inspection will start after Easter and continue through to the autumn.
The agency said it wanted to ensure the bridge was still in good order and to find out what maintenance would be required to keep it in sound condition in the future.