THE new chief engineer of the Forth Road Bridge has given the clearest indication yet that the corrosion threatening the structure's safety can be halted and repairs carried out to keep it open for 100 years.

In a meeting with a campaign group opposed to a new Forth road crossing, Barry Colford, who took over as bridgemaster in February, said he was highly confident that cable drying, now under way at a cost of £10.3 million, would halt the decline in the strength of the bridge.

He told the ForthRight Alliance: "Although the main cable has lost about a tenth of its strength since construction, we now have a high degree of confidence that the deterioration can be arrested by dehumidification." Last year's campaign for a replacement Forth road crossing was based on claims that corrosion might force early closure of the bridge.

The confirmation that cable drying is expected to succeed is being interpreted by transport campaigners as an indication that cable replacement will not be needed and that a new bridge will be unjustified.

Paul Tetlaw, chairman of the sustainable transport campaign, Transform Scotland, said it was clear that there was now no reason to go ahead with building another road crossing.

"This is good news indeed for all those concerned about road transport across the Forth," he said. "We should now be able to move forward from the scaremongering about the need for an additional bridge and look at how we can more wisely spend £4.2 billion of public money."

Then Forth Estuary Transport Authority (Feta) which maintains the bridge, said: "The bridge is still corroding, but the corrosion is progressing more slowly than we originally feared. Engineers will look at it again in 2012 to verify how effective the dehumidification has been."

On its website, Feta adds: "The good news is that, although the cables are still losing strength, the worst-case scenario of an HGV ban in 2014 now looks unlikely." And the spokesman told the Sunday Herald: "The rate of deterioration appears to be at the more optimistic end of previous predictions, and the earliest a weight restriction might be needed is about 2017."

Lawrence Marshall, a former convener of Feta and now acting chairman of the ForthRight Alliance, urged the government to think again. He said: "Instead of spending £4.2 billion on an additional crossing, the Scottish government should earmark £100m to ensure that the cables on the existing bridge can be augmented or replaced in case dehumidification fails to arrest corrosion. If required, this would be a reasonable expenditure to safeguard this vital road link."

Significant problems remain for the government as it seeks to justify a new bridge against opposition from transport and environment campaigners. Several ForthRight Alliance members, including Friends of the Earth Scotland, have pointed out the inconsistency of such a large project with the government's environmental policy. Lawrence Marshall says all the evidence suggests a new bridge won't alleviate congestion. He added: "Traffic research shows it would effectively double the flow of traffic across the Forth, with attendant increased emissions, so we cannot see how it is compatible with the SNP government's pledge of an ambitious programme to tackle climate change."

Katharine Taylor, of Sustrans, said: "We would much rather see improvements to public transport and investment in high-quality walking and cycling routes to improve people's health and the environment."

However Transport Scotland, the national transport agency, said plans for another crossing would go ahead. A spokesman said: "It will be 2012 before we get a definitive picture about the state of the bridge, and so nothing has really changed."

A recent report by Edinburgh council's transport department suggested the case for a new Forth bridge had not been made.

Meanwhile, exploratory works are now under way on the bed of the river Forth, and as the area prepares to be turned into a building site for a decade, some residents of nearby South Queensferry who supported a new crossing are beginning to reconsider their decision.