A new inspection has revealed weight restrictions on the Forth Road Bridge may not need to be applied until some time between 2017 and 2021.
Engineers had previously feared that corrosion to the main cables could require the bridge to be closed to heavy lorries as early as 2013 and to all vehicles at a later date.
The latest inspection has shown the main cables have lost about 10 per cent of their strength through corrosion, said the Forth Estuary Transport Authority.
An inspection in 2004 revealed they had lost about 8 per cent of their strength and could continue to deteriorate to the point where weight restrictions could be needed some time between 2014 and 2020.
An independent audit commissioned by the Scottish Executive confirmed this - and suggested weight restrictions might need to be introduced as early as 2013.
But the latest inspection, carried out in February and March, put the rate of deterioration "at the slower end of expectations," said FETA.
It said: "If efforts to halt the corrosion fail, it is now thought more likely that weight restrictions might require to be considered some time between 2017 and 2021."
A dehumidification system is already being installed with aims to prevent further corrosion by drying out the cables, and a third inspection will take place in 2011 to check the effectiveness of this.
The work is being carried out by engineering consultancy Faber Maunsell.