Today, the board heard the odds of a hit on the navigation columns — those on which port and starboard lights are located for ships travelling up and down to Perth — were sufficiently high for protection to be introduced.

In 1983, the bridge came perilously close to disaster when the 688-foot tanker Chelsea drifted onto a sandbank just 200 yards from a supporting column. Traffic was cleared from the bridge as tugs and an oil support vessel scrambled to pull her to safety.

A year later, a German coaster struggled for hours to pump water from her holds after a collision with the bridge tore a hole in her hull, and in 1998 the Omega 3, carrying a cargo of animal feed to Perth, smashed into a column in reduced visibility and was badly holed.

Bridge engineer Ken Laing said a column-protection system would probably take the shape of pontoons, of the type that protected the Forth Bridge columns.

He said the figure of £3 million was in the 20-year plan for the bridge, but it was difficult to put an exact figure on the costs of constructing a pontoon barrier.

Meanwhile, it looks likely that lane-restriction rage for motorists using the south bound lane of the bridge will start in May, when they will have to endure for the next three years a £12.5 million programme of repairs.

Mechanical bearings at the top of the supporting columns need to be replaced and, while the work will be out of sight of the public, the left lane south will have to be progressively closed to allow safe access for workmen.

Mr Laing said there had been a number of technical difficulties in the design of the bearing replacements, since when they first built there was no allowance for future maintenance.

The work entailed taking the top off the columns and removing bearings, recasting the top of the columns, taking out the old bearings and putting in the new ones.

The lane restriction will begin at the tolls and gradually progress south, although Mr Laing said the works would stop throughout winter.