HUGE £35million drainage works have been put on hold after tram lines were discovered under a road.

The first part of the project in Blackburn and Darwen was due to begin next month but now bosses at United Utilities say it will be delayed for at least three weeks.

The scheme, aimed at cleaning out the rivers in the two towns, will see miles of pipes replaced and five underground overflow tanks installed.

Work underneath Bolton Road, Blackburn, has been put on hold due to tram lines below the road’s surface interfering with state-of-the-art sonar equipment which was being used to pinpoint pipes and electricity cables.

A spokesman for the firm said engineers are having to resort to an old fashioned technique of digging trial holes in the road in order to locate pipes and cables.

United Utilities said engineers must conduct further trials to ensure they can carry out works safely and efficiently.

Ewood councillor Maureen Bateson said: “If you look at old pictures of the Empire Theatre you can see the tram lines so I’m surprised they didn’t know about them already.

“We all knew that a lot of unexpected things were going to be found underneath Bolton Road.

"I’m surprised that United Utilities were as confident as they were that it was going to be a simple job.”

The delay comes amid fears that the works will cause major disruption in the area.

United Utilities said that no work will begin until letters have been sent to Bolton Road residents and businesses.

A spokesperson for United Utilities said: “We initially told people about plans in order to ensure they were given plenty of notice.

"Unfortunately, the tram lines have caused problems with our sonar scanning equipment, giving us blank readings.

"This means works originally scheduled to commence in early May will have to be delayed by at least three weeks.”

The project is scheduled to run until October 2014 on roads across the borough.

A section of the busy Bolton Road is set to be closed for at least two months, and a car park off Aqueduct Road will be unavailable when the work gets underway.