C Spencer completed a £7.8m de-humidification job on the bridge last September designed to prevent moisture causing problems in the bridge’s two main cables.

But the Forth Estuary Transport Authority has confirmed that more investigation work will be needed on the bridge’s anchorages to ensure moisture has not seeped into the chambers and corroded the steel strands.

The authority is now looking for a contractor to carry-out the work on the South Queensferry side of the bridge which will begin next year.

Authority chief engineer and bridgemaster Barry Culford told The Courier: “Our intention now is to press ahead with the investigation. It is the only way to deal with long-term concerns over the structural integrity of the anchorages.

“Excavating down to expose the steel will allow us to assess the current strength of the anchorages and will help in the difficult task of estimating future strength.

“Depending on the findings, challenging engineering decisions may have to be taken. But it must be stressed there is no direct or current evidence the bridge is in any way unsafe.

“Frequent inspections are carried out to monitor for movement within the anchorage chambers and no signs of distress or movement have been recorded to date.”

A further inspection of the two main cables will also be carried out in 2012 to check the effectiveness of the de-humidification work.