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."