United Utilities has picked the joint venture ahead of four other bidders for the project.
The ambitious scheme will involve laying and relining more than 100km of pipes to supply water to homes and businesses from Thirlmere Reservoir.
The scheme is designed to ease pressure on environmentally sensitive local water sources in West Cumbria, and provide a secure, long-term supply for the area.
It has been broken down into several work packages.
Roadbridge and Farrans will deliver 32km of large diameter twin water mains and 24 km of large diameter water mains.
The £300m project will also involve the construction of new water treatment works, two service reservoirs and a new pumping station.
The pipeline’s route from the reservoir will loosely follow the direction of A591 northwards through Keswick, passing east of Bassenthwaite, before turning due west towards Williamsgate.
From Williamsgate treatment works a new treated water trunk main will deliver water to Summergrove service reservoir near Whitehaven.
United Utilities will start construction of the major four and a half year project shortly.