Restrictions are to be brought in on heavy goods vehicles (HGVs) using the old Severn Bridge due to corroding support cables.
The bridge carries a footpath and cycle track Pic: Severn River Crossing PLC
They will be barred from using the outside lane in both directions from November as £3m tests are carried out.
The inspection is being paid for by owner the Highways Agency, and Seven River Crossing, which operates both it and the newer bridge down river.
The Highways Agency reassured motorists that the crossing remains safe to use.
The mile-long bridge was opened to traffic in 1966.
The agency said it became concerned after similarly-designed crossings around the world were showing signs of corrosion.
Examination work began in April on the cables. The main cables are 511mm (20ins) in diameter and each one is made up of more than 8,300 individual wires.
The agency does not have to replace the cables, but only those wires that are corroding or snapped.
SEVERN BRIDGE FACTS
The bridge is 1,597m (5,240ft) long
Its two towers stretch approx 122m (400ft) above the river
The sound of lasers in the film Star Wars was made by striking one of the bridge's suspension wires
The original bridge has carried more than 300m vehicles since it opened
It was after the Forth Road Bridge in Scotland was found to be corroding that tests were carried out on the old Severn crossing.
The agency intends to set up a dehumidifying system where the cables are encased in a rubber-like substance and the space in between is filled with a dry air which is constantly pumped around. This work would cost £25m.
The system is being trialled on the Forth bridge - a year ahead of Wales in the repair process.
The 40th anniversary of the opening of the first Severn crossing was celebrated last month. It was opened to traffic by the Queen on 8 September 1966.
It was built to replace the ferry service that crossed from Aust Cliff in south Gloucestershire - near the Welsh border - across the Severn to Beachley Peninsula in Gloucestershire.
It was strengthened in the 1980s to cope with the growth in volume of traffic, but was replaced as the main route into south Wales in 1996, when the second Severn Crossing was opened downstream.
The Severn Bridge, which took three-and-a-half years to build is actually comprised of two bridges - the main section spanning the Severn and the other crossing the River Wye.