THE Whalley Arches railway viaduct is to undergo major repairs after part of the structure started to sink into the ground.

Wooden supports on one of the 49 arches have started to rot causing the 1846 structure - the largest of its kind in Lancashire - to subside Network Rail engineers are to take out the rotting wooden pillings on arch 39 near King Street and insert steel replacements.

Engineers have been using remote sensors since February to monitor the scale of the problem.

Repairs on the Grade ll-listed structure must be agreed by English Heritage before they go ahead.

Details of the cost of the project have not been revealed.

Once approved, rail bosses hope the work will be completed by next spring.

Network Rail spokesman Keith Lumley said: “It's not something serious enough that it needs to be tackled now and could stop the train line running.

“Train services on the viaduct have a 45mph limit and that's perfectly acceptable for the type of distortion there is in the viaduct.

"Long term if we don't do anything about it then yes it would become a much more serious problem."

Engineers will have to build a scaffold structure around the viaduct before they spray liquid concrete into a cage underneath the arch which will be removed once the concrete has set.

Mr Lumley added: "A very similar problem occurred in 1940 when the wood began to rot and the peers had to be underpinned."

The viaduct, built by the Blackburn and Bolton Railway Company provided a vital lifeline for small rural communities, linking Clitheroe and Blackburn.

It is the longest brick viaduct in Lancashire at 605 metres and at its highest point it is around 21 metres high.

The 7million bricks used to build it were made in Whalley.