The rail operator wants work to start on site by the end of this year on the refurbishment job which is expected to last 36 months.

The contract represents the biggest renovation scheme on the 151-year-old structure which still carries more than 30 trains a day.

The most complex plan ever to refurbish the Royal Albert bridge has been finalised and Network Rail will soon be tendering for the contract to carry out the improvement work.

Corroded elements on the bridge will be repaired and extra efforts will also be taken to restore Brunel’s original design. These include removing redundant bridge elements – lower diagonal bracings – installed in the 1970s and using special bolts that are similar in design to Victorian rivets.

The rail landmark and gateway into Cornwall will also be spruced up with new paintwork covering a total area of 20,000 square metres – about three football pitches. Thirty coats of paint on the main spans will be blast removed before being painted over, using a more effective three-coat painting system.

The Royal Albert bridge will be painted goose grey, so that it is compatible with the colour of the bridge when it was listed as a Grade 1 structure in 1952 by English Heritage.

The effort to improve the bridge has also helped trace the bridge’s original colour of pale stone (off-white) that it was painted in 1859, replacing the missing piece that completes the jigsaw of Royal Albert bridge’s colourful history. Two 1m square patches of the existing paint layers on the bridge will be preserved for posterity.

The discovery was made from an unprecedented paint analysis commissioned by Network Rail. It is to help Network Rail’s engineers understand the bridge’s existing complex paint system and define an effective approach to repaint the structure’s two main spans.

Chris Rayner, route director, Network Rail said: “”This effort has affirmed a piece of Royal Albert bridge’s history that has been obscured under 30 coats of paint for more than a century. It will also be the first time that the colours of the bridge is officially chronicled for posterity.

“Most importantly, the industry has an ambition to transform the railway on Great Western in the coming years and our investment on the Royal Albert bridge will play a part. It will be the most complex refurbishment work since the bridge was built and will inject a new lease of life into this vital rail link, keeping it robust for many years to come.”

The Royal Albert bridge has been painted over on 20 occasions since it was completed in 1859. However, its parapets and main spans were repainted in red-brown for £1,700 in less than a decade.

In 1868, the ‘I K Brunel Engineer 1859’ lettering on the portals of the two main spans was painted at a cost of £2.31.