An extensive inspection of the M48 Severn bridge will start this month, after signs of corrosion were found on its sister bridge in Scotland.

The bridge, which is 40- years-old in September, will have its main suspension cables cut open to see if it shares any of the defects found on the Forth Road Bridge in Scotland, which was built around the same time. The work will be carried out by the Highways Agency, who are confident it will be completed by October.

Highways Agency senior bridge engineer Martin Lynch said: "The inspection will enable us to examine inside the main suspension cables as the bridge nears the 40th anniversary of its opening. We are taking a precautionary approach in the light of a similar inspection which found corrosion in cables on the Forth Bridge.

"There are no concerns for the safety of people using the bridge at this stage, but the results of the inspection will enable us to decide what future maintenance will be required to keep it in good order."

The cost of the inspection work is £3m, which has been agreed by the Highways Agency and Severn River Crossing plc, the concessionaire for the M4 and M48 Severn bridges.

Severn River Crossing plc will pay £300,000 of the work, with the goverment footing the rest of the bill.

The M48 bridge was opened by Her Majesty the Queen on September 8, 1966. Six men died during its construction.

The main cables which are going to be cut open are 511mm in diameter and each one is made up of 8,322 individual wires.

The inspection will not cause an increase in toll charge to drivers and it is hoped the public will not be greatly affected by the project. Mr Lynch said: "During the work, there will be scaffold platforms built around the sections of cable being inspected and for the safety of drivers and workers, a 40mph speed limit will be in place while work is being carried out. "There will also be some overnight closures when the scaffold platforms are moved to new locations. However, the work will be designed to minimise delays to drivers."