The Consumer Council for Water has questioned the affordability and acceptability of the proposed £2bn Thames Tideway tunnel.
The 32km tunnel, to collect overflows from London's sewers, will run beneath the Thames. It would be, the UK's largest single sewerage project since water privatisation in 1989.
There is no planned public subsidy for the project so the cost will eventually be carried by all Thames Water sewerage customers.
Thames Water plan a 7.2m diameter tunnel, which will be dug as far as 80m below ground level, testing tunnel boring machinery to its limits.
The Consumer Council for Water believes that the cost raises serious concerns about customer acceptance and affordability.
National Chair of the Consumer Council for Water, Yve Buckland said, "This is a massive project by anyone's standards and with all the associated risks and potential for costs to increase over the life of the work.
"But the big issue here is the cost, and what will eventually be added to sewerage bills is way above what customers are willing to pay.
"Not only that, the increases will mean that low-income customers will be hit particularly hard, and they will simply find their sewerage bills unaffordable."