A £2bn tunnel is to be built to prevent untreated sewage and rainwater polluting the River Thames.
Thousands of fish have been killed in the Thames
The 20-mile (32.2km) tunnel will catch sewage and rainwater discharges along the length of the river and carry it to be treated in east London.
The tunnel will add about £37 to each Thames Water bill by 2017, water service regulator Ofwat has said.
The tunnel hopes to catch the 52 million cubic metres of waste which pollute the Thames and Lee annually.
It is expected to be completed by 2020.
London's sewer network, built in the 19th Century, collects sewage and rainwater together but it can no longer cope with the city's growing population and change in climate, the Environment Agency has said.
The untreated discharge kills fish, pollutes the river and damages the Thames' environmental and visitor appeal, the Department for Environment, Food and Rural Affairs added.
Environment Minister Ian Pearson added: "This tunnel is the right solution for London and for the environment. It will give us a 21st Century River Thames that we can all be proud of.
"This will inevitably mean some extra costs for customers, but it will deliver tangible benefits for London in the long term."
But water services regulator Ofwat said that it had concerns about the cost to Thames Water customers.
20 miles (32.2km) long
Seven metres (23ft) wide
Runs from Hammersmith to Beckton
There will be a 3.4 mile (5.5km) long spur tunnel from Abbey Mills (east London) to Beckton
"Ofwat remains concerned about the value for money for water customers of this very costly scheme - probably over £2bn - in terms of the health and environmental improvements it will deliver," a spokesman said.
David Bland, chairman of Consumer Council for Water Thames, said: "The costs will be met wholly through increases in consumers' bills, and will be far more than most are willing or expecting to pay.
"For some Thames Water customers, the resulting price rises will be simply unaffordable".
Thames Water's chief executive officer, David Owens, said: "We will now focus on delivering this massive and challenging engineering project as efficiently as possible, to minimise the impact on our customers' bills."