A £4.2 billion contract for the Thames Tideway Tunnel has successfully been completed.
The Government Legal Department (GLD) said the milestone brings the UK “a significant step closer” to modernising London’s Victorian sewerage system.
It follows five years of negotiations by lawyers at the Department of Environment, Food & Rural Affairs (Defra) through “many of the obstacles that threatened the project”.
The so-called “super sewer” is expected to stop millions of tonnes of sewage that currently overflow into the Thames every year.
The lawyers also developed a regulatory framework, which is said to have “significantly” reduced the expected cost to Thames Water customers.
The project is now said to add no more than £25 extra a year to water and sewerage bills by the middle of the next decade compared to previous forecasts of up to £80.
Alan Evans, GLD’s Legal Director at Defra, said: “The Thames Tideway Tunnel is many times larger than any previous project in the water sector, with a price tag of more than £4bn and the added complication of digging under the entire length of a major capital city.
“There is a huge sense of professional pride in knowing that we have been critical in ensuring the go-ahead of this world-leading infrastructure project. A project that will boost economic growth, generate more than 9,000 jobs and bring huge benefits to the natural environment.”
Construction of the tunnel is expected to start next year and take seven years to complete.