United Utilities has called for contractors to register their interest in a £1.75bn deal to replace a 109km water pipeline in the North West.
The organisation has published an official contract notice for delivery of the Haweswater Aqueduct Resilience Programme.
The successful bidder will design, build, finance and maintain the replacement for the 67-year-old asset, which takes water from the Lake District to Greater Manchester.
Engineers started inspecting the aqueduct in 2005, and identified areas of concern that posed a potential future risk to water quality and supply.
All six sections of tunnel will be replaced and fresh connections will be made to existing multiline siphons.
The selected contractor will also be responsible for maintaining the new infrastructure for 25 years after the works are completed.
Firms have until 17 August to express their interest, with three parties expected to be shortlisted.
The Haweswater Aqueduct Resilience Programme is broken into six stages, covering each of the tunnel sections. Webinars and virtual exhibitions were held last year to showcase the details.
Separate planning applications have been made for each of the six tunnel sections and decisions are awaited.
Costain won an early involvement contract on the programme in 2019, covering assistance with preliminary designs, land surveys and investigations.