Extra-wide hard shoulders planned for the new Forth Road Bridge are to add tens of millions of pounds to its cost.

Work will begin next year on the new, £2 billion bridge, amid fears over corrosion on the current crossing.

However, a report published by the Scottish Parliament Information Centre revealed the cost would be higher because its hard shoulders have to be a yard wider than normally needed to cope with high winds. The report says that the bridge will be the most expensive major project per lane in the world.

Cables on the existing crossing are corroding, and it was revealed last year that traffic would have to be restricted by 2017 without remedial work. That is now under way - but its success will not be known until 2012.

The new bridge will allow the existing one to be suit, repaired and then reopened for public transport only.

The report states: "These hard shoulders are wider as they are designed as full running lanes to accommodate public transport when the existing Forth Road Bridge is closed during high winds, and are future proofed to provide a light rail corridor should this be necessary.

"This enhanced provision has structural consequences which impacts significantly on to the cost."

Green MSP Patrick Harvie attacked the latest news, telling Scotland on Sunday: "This deeply unpopular scheme is shaping up to be the biggest mismanagement of Scotland's finances in at least a generation."