BUILT to cater for the motoring boom of the 1930s, the Kincardine Bridge was considered a marvel of modern engineering.

Through today's eyes, it may appear a bleak and purely functional structure, but it has just been granted the highest level of statutory protection in recognition of its "national and international importance".

The Kincardine Bridge, the first crossing built over the Firth of Forth, is today awarded category A-listed status by Historic Scotland, which is responsible for protecting Scotland's built heritage.

It now enjoys a similar status and level of protection as sites such as Edinburgh Castle, Glasgow Cathedral and Marischal College in Aberdeen.

Built between 1932 and 1936 to a design by Sir Alexander Gibb & Partners - the civil engineering practice which oversaw the design of dams, roads, bridges, and harbours in Europe, Africa, and the Middle East - the bridge spans the Forth at Kincardine, 10 miles south-east of Stirling.

When it opened to traffic in 1936, it was the largest road bridge in the UK. It now carries the A876 from Fife to the M876 and M9 motorway networks on the southern side of the river.

Originally designed as a swing bridge, its 364ft span was the largest swing span of any bridge in Europe at the time.

Operated from a control cabin in the gantry above the bridge, the central swing section would open to allow large ships to pass upstream to Alloa.

As well as being an engineering wonder of the age, the bridge was also at the forefront of pioneering science.

The exact location of the swing was controlled to a precise degree by a photoelectric cell, the first time the new technology had been applied in this way.

It was so accurately set that the 1600 tonnes of swing span could be turned through 90 degrees by just three farthingsworth of electricity, less than 1p today.

The original swing mechanism was so good it operated without fault until it closed for the last time in 1989, when Alloa ceased to be a port.

The bridge was constructed at a time when cars were becoming increasingly popular, but people travelling between Edinburgh and Fife had been forced to go as far west as Stirling or use a ferry at Alloa or Queensferry to cross the Forth.

The bridge is due to undergo significant renovation work to ensure that it continues to function effectively into the future, after a planned second crossing is in place.

The proposed new bridge will cross obliquely from near the southern end of the existing bridge to the old site of the Kincardine Power Station before taking traffic to the west of Kincardine-on-Forth, the town that gave the old bridge its name.

A spokeswoman for Historic Scotland said: "The listing of Kincardine Bridge comes at a time when there is an increasing awareness of the need to celebrate and protect the architectural and engineering achievements of our more recent past as well as the old."

Patrician Ferguson, minister for tourism, culture and sport said: "The bridge is a fantastic example of Scotland's innovative architectural past.

"By protecting this wellloved landmark structure we continue to raise the awareness of the importance of architecture and design to the cultural richness of our country."

There are more than 46,000 listed buildings in Scotland, but only 7.5-per cent are considered important enough to be on the Category A list.

The Forth Bridge was category A listed in 1973, making it the largest A-listed structure in the UK. In 2001, the Forth road bridge received the same classification.