The Thames Tideway Tunnel is now 40% built and is on track to be completed by its 2024 deadline.
Tideway’s infrastructure project, which will run from Acton in west London to Stratford in east London, will boost the capacity of the city’s Victorian sewer system and reduce the amount of sewage leaking into the Thames.
Five years after the project was given the green light, more than 2,000 people are working on the 25-kilometre tunnel.
With the unpredictable initial tunneling phase now complete, the company has increased its estimate of total costs by 8% to a total of £3.8 billion, up from £3.52 billion or £3.14 billion in 2014 prices.
There will be no change to the estimated £20-25 annual cost for Thames Water bill-payers and the company stresses it has already raised enough money to complete the project without requiring additional funding.
Andy Mitchell, Tideway’s CEO, said: “As we approach the half way point of construction, the time is right to update our cost estimate.
“The most important thing to say is that there will be no impact on the estimated cost to Thames Water bill payers and to recognise that our teams have done a great job in getting us this far and keeping us on schedule. Every day we get closer to our aim of giving London a cleaner river.”