The Forth Road Bridge was built 40 years ago

Water damage inside the cables is making them weaker, and shortening the life of the bridge.

The �12m project will dry the cables out and could slow down the rate of corrosion.

The installation work is due to start next April, at the same time as four months of weekend roadworks to resurface the northbound carriageway.

Over a typical day, the bridge carries around 180,000 tonnes, over half of which is derived from HGVs Barry Colford FETA's depute general manager

Barry Colford, depute general manager of the Forth Estuary Transport Authority, which operates and maintains the bridge, said: "Unfortunately, the need for resurfacing now arises much more frequently than it used to, due to increased traffic volumes and heavier vehicles causing ever-greater wear and tear on the bridge.

"The southbound carriageway was resurfaced in 2004, and now we are beginning the process to appoint a contractor for the northbound carriageway.

"Increased loading, especially from HGVs, puts tremendous pressure on the bridge's surfaces.

He said that in a typical day, the bridge carries about 180,000 tonnes, more than half of which is derived from HGVs.

"Such increased usage has been compounded by an increase in the HGV weight limit to 44 tonnes and a move to super-single high-pressure tyres, which all adds to the damage caused to the bridge's road surfaces," Mr Colford said.