Thames Water pumped 14bn litres of sewage into river Thames in central London in 2023

Thames Water pumped 14.2bn litres of sewage into the river Thames in central London last year, according to data that highlights the quantity of effluent flowing from the pipes of the UK’s largest water company.

The figures, obtained under freedom of information laws, reveal for the first time the volume of sewage outflows into the section of the river covered by the £4.5bn Tideway Tunnel, which is due to open next year.

They also come days after Sir Chris Whitty, England’s chief medical officer, warned that sewage pollution in UK waters had become a “public health priority” that needed to “be taken seriously”.

Water companies are facing a growing backlash against sewage pollution, which has risen up the political agenda in marginal seats in the south of England that the Conservatives are seeking to retain in the election on July 4.

Thames Water, the UK’s biggest water provider by number of customers, says the Tideway Tunnel project, which began eight years ago, will significantly cut the amount of sewage leaked from its network.

While private water companies are obliged to monitor and report on the number of sewage overflow incidents, Thames Water has electronic devices that measure the quantity of effluent flowing into the part of the Thames that falls within the Tideway Tunnel area of central London.

These devices recorded at least 85.9bn litres of sewage being pumped into that part of the river between 2020 and 2023, according to freedom of information requests obtained by the Liberal Democrats over successive years.

The latest, released this month, showed that 14.2bn litres of sewage entered the Thames at these sites last year.

Sarah Olney, Treasury spokesperson for the Lib Dems, said the outflows last year were “disgusting” and called for every combined sewage overflow pipe to be installed with monitors measuring volume and frequency. 

The worst incident took place in October at Mogden in south-west London, where 558mn litres of sewage were discharged.

The second-worst incident occurred at Crossness in Bexley, where 430mn litres of effluent were released in one day in November. Both Mogden and Crossness are home to sewage treatment works.

The increased focus on sewage overflows has also increased public and political scrutiny of water companies’ finances.

The 16 water and sewage companies in England and Wales paid out £78bn in dividends and accrued £62bn in net debt between privatisation in 1991 and March 2023, according to research by the Financial Times.

Ofwat was due to rule in June on how much water companies would be allowed to increase bills in England and Wales until 2030, but the sector regulator has delayed its decision until July 11 because of the election.

The decision will have a bearing on the financial health of Thames Water, which is struggling under the weight of its £18bn debt mountain and needs a £750mn cash injection from shareholders this year to keep running and deliver infrastructure improvements.

Ministers have already drawn up contingency plans to put the business into special administration, a form of temporary nationalisation.

The Lib Dems are calling for special administration measures to be enacted. Labour, the main opposition party, is urging a regulatory shake-up.

Thames Water said retrofitting the entire network with volume monitors would be “prohibitively expensive”. It said it had published plans to upgrade 250 sewage treatment works and sewers including a £100mn upgrade of the Mogden sewage treatment works and a £145mn upgrade of the Beckton sewage works.

“We regard all discharges as unacceptable and taking action to improve the health of rivers is a key focus for us,” the company said, adding that the Tideway Tunnel would capture 95 per cent of the volume of untreated sewage entering the tidal section of the river in a typical year once operational. 

Additional reporting by Jim Pickard