DIRE predictions the Forth Road Bridge could be forced to close to lorries by 2016 have been dispelled by new tests showing the crossing could still have decades of useful life.

The latest examination of the main cables of the suspension bridge found a dehumidification system installed in 2010 had succeeded in arresting further corrosion, prompting fresh criticism of the decision to plough ahead with a £1.6 billion replacement crossing.

A report prepared for a board meeting of the Forth Estuary Transport Authority (Feta), detailed the results of the first tests carried out since the new system was activiated.

Bridgemaster Barry Colford said: "The rate of deterioration of cable strength has been reduced and the factor of safety against failure of the cables has not materially diminished."

A spokesman for Feta said the bridge had lost 10% of its strength when tested in 2008 and this had not worsened significantly by last year.

Alison Johnstone, Green MSP for Lothian, said: "This report removes the central justification the SNP made for blowing over a billion pounds on a new bridge. The Scottish Greens were the only party arguing strongly that repairing the existing bridge would have cost a fraction of the price"

However, Transport Scotland, the government agency overseeing construction of the replacement crossing, due to open by 2016, defended the investment.

A spokesman said: "A degree of uncertainty will remain. Damage to the wires inside the cables cannot be repaired."