The 4.5m diameter tunnel will be driven at depths of between 10m to 30m and will run through an area of old mine workings raising potential technical problems.

Two designs are currently under consideration, a shallow tunnel that would cross major infrastructure road and rail routes or a deeper tunnel within stable bedrock that would need a lift pump station.

The winning firm will need to connect the tunnel at both ends to the existing sewer network.

It is anticipated the project from contract award to finish will run for seven years.

Expressions of interest must be submitted by e-mail to [email protected] by 10 December.