United Utilities has invited firms to pitch for work valued at up to £500m as the FTSE 100 company pushes on with a multibillion-pound programme of investment in its water networks.

The company is looking to hire firms to advise on its 2015-20 capital investment programme, which has come in the wake of a raft of regulatory changes in the water industry.

More than £3.5bn has been set aside for investment in the company's water and wastewater services, with the related legal work expected to be worth £100m-£500m.

Successful law firms will advise on the design, building and operation of United Utilities' water network, as well as its future maintenance and financing.

In the past the company has turned to Addleshaw Goddard, Eversheds Sutherland and Slaughter and May.

Earlier this year United Utilities agreed a joint venture with Severn Trent, dubbed Water Plus, in the wake of the introduction of new competition rules in the water sector. A legal panel was set up to support the new business, with Bond Dickinson, Shepherd and Wedderburn and Pinsent Masons appointed to the line-up.

Slaughter and May advised United Utilities on the joint venture. Eversheds Sutherland is the sole legal adviser to Severn Trent.

United Utilities' legal team is headed up by general counsel and company secretary Simon Gardiner, supported by head of legal David Hannon.