The chief executive of the London “super sewer” project has been awarded a £600,000 pay rise even as the firm revealed that the total cost had risen by £100m.

Andy Mitchell received pay of £2.5m for the year to March 2025, up from £1.9m the year before, according to accounts published by the Thames Tideway tunnel builder, Bazalgette Tunnel Limited.

The Tideway project is seen as a crucial upgrade to London’s sewage system, which had been reliant on sewers designed during the reign of Queen Victoria by Joseph Bazalgette. The sewer was a response to the “Great Stink” of 1858.

London’s population has more than tripled since then, and water pollution has again become politically salient because of widespread disgust about sewage leaking into Britain’s rivers and seas. That has led to close scrutiny of executive pay in the water industry, which is run as a series of privately owned monopolies.

The 15.5-mile sewage tunnel runs mostly under the Thames. It was originally due to be completed in 2024 but was delayed by the Covid pandemic. The total cost has now reached £4.6bn, the Tideway company said, up from £4.5bn reported last year and more than £1bn over the initial £3.5bn estimate.

Mitchell’s pay increase was mostly driven by £500,000 added to his “retention bonus”, which hit £1.5m. He received a base salary of £550,000, plus an extra bonus of £423,000 and unnamed “benefits” and pension contributions worth £21,000.

However, his total pay was lower than the £2.7m he received in 2023. Mitchell has previously worked on projects including Hong Kong airport and the Thameslink and Elizabeth rail lines in London.

The scale of the rewards on offer have been criticised, given that the Tideway company is funded by mandatory additions to customer bills.

Luke Hildyard, the director of the High Pay Centre, a thinktank that tracks executive pay, said: “The culture of very high top pay and extreme concentration of income at the top of the distribution in the UK is a real problem for living standards. If so much of the wealth generated by our economy is captured by a tiny elite, it becomes much harder to improve life for everybody else.

“We need a plan to address this issue and more reasonable pay levels for quasi public service roles like this would be an obvious place to start. No doubt running Tideway is a challenging role, but it also carries considerable prestige and other non-financial rewards. It seems likely that a competent project manager could have been found without requiring such extravagant executive pay awards.”

Matthew Duncan, the Tideway company’s finance director, received pay worth £1.7m, up from £1.3m last year.

A Tideway spokesperson said: “The Thames Tideway tunnel is a world-class infrastructure project and one of the most complex in the UK. It has been delivered within the original timetable and within the cost range estimate for bill payers outlined before we started work, at around £25 a year. After nine years of construction and the work of around 25,000 people, it is now operational, protecting the River Thames from millions of tonnes of sewage pollution.

“Tideway’s remuneration reflects the complexity of the engineering challenge and takes account of schedule, cost, health and safety and sustainability goals. The increase is largely due to the payment of a long-term retention incentive that has supported consistent leadership and the successful delivery of this vital infrastructure project.”