Tideway has revealed the cost of building’s London’s new “super sewer” has increased by around 8 per cent.

In its latest update, the firm said the revised cost of the project now stands at £3.8 billion, compared to the original estimate of £3.52 billion, after completing the “first and most unpredictable” phase of the work.

But the company added there will be no change to the estimated £20-£25 annual cost for Thames Water bill payers as “costs remain will within the original projection for customer charges”.

The update also shows that 40 per cent of the construction work on the new sewer is now complete and the company is on track to deliver the project on time by 2024.

“As we approach the half way point of construction, the time is right to update our cost estimate,” said Tideway’s chief executive, Andy Mitchell.

“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.”

When finished, the tunnel will run from Acton in west London to Stratford in east London.

Construction is taking place at 23 sites across the city and Tideway is using the river instead of roads as far as possible to build the project, cutting the impact on road congestion and air quality.

By the end of April 2019, one million tonnes of material will have been moved by river – keeping 60,000 HGV movements off London’s roads so far.

Earlier this year, Tideway announced it had avoided the need to divert a Victorian gas main at Blackfriars, one of its most complex sites.

The diversion would have meant a closure of the Embankment to traffic for up to six months. To avoid the diversion, Tideway had to make fundamental changes to the construction methodology for the site, adding costs to the original estimate.

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