It argued that no public subsidy is planned and customers of Thames Water could foot the £2 billion cost. National chairwoman Yve Buckland said: "What will be added to bills is way above what customers will pay."
A Thames Water spokesman responded: "There are financing issues, but a cost has not yet been agreed."
The tunnel is designed to help London's strained sewer system. It is due to be completed by 2020 (Planning, 30 March, p5).