Tideway said 40% of the work is now complete after the “first and most unpredictable phase of the project” was now finished.

The company’s revised cost estimate is now £3.8bn compared to £3.52bn originally which represents an 8% increase.

Andy Mitchell, Tideway’s CEO said: “To build the super sewer we are working in an unforgiving tidal river in the centre of one of the busiest cities in the world. We have got train lines and bridges above our heads, and the tube and utility pipes below our feet.

“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.”

Complication have included the need to divert a Victorian gas main at Blackfriars where “fundamental changes to the construction methodology for the site” have increasees costs on the original estimate.

Costs also increased at Tideway’s King Edward Memorial Park site in Wapping where the team found difficult and unexpected ground conditions which meant they have had to do significant work to reinforce the ground to make it stable for construction.

Tideway also took the decision to reduce HGV movements, which required additional investment in the infrastructure needed to move materials by river, as well as investment in skills and jobs through the new Thames Skills Academy.

Tideway will publish its full annual report and accounts in June.