It serves a population of 15.5 million people—over a quarter of England's population—but its ageing infrastructure is prone to leakage and is a frequent cause of pollution, for which it has been repeatedly prosecuted and fined.

To resolve these issues, the Metropolitan Board of Works, under the leadership of Chief Engineer Joseph Bazalgette, constructed a large network of sewers by 1870, many of which are still in use today.

[16] On 22 March 2017 a record fine of £20.3m was imposed on Thames Water after large leaks of untreated sewage, totalling 1.4bn litres, occurred over a number of years.

[21] This situation had been recognised by the regulator Ofwat in December 2022 - leading to increasingly urgent discussions concerning a possible collapse and potential state bail-out of Thames Water.

[26] On 29 June 2023, with the UK government reportedly on standby for a potential taxpayer bailout through a temporary nationalisation,[27][28] with pension funds worried about their investments in the firm,[29] and with suppliers engaged on major projects concerned about payment,[30] Thames Water announced experienced City troubleshooter Sir Adrian Montague would lead rescue efforts, succeeding Ian Marchant as chairman on 10 July 2023.

[42] Responding to the request, Michael Gove, the UK's housing and communities secretary, stated that "Thames Water leadership has been a 'disgrace'" and customers should not be expected to pay higher bills.

[43] In April 2024, The Guardian reported Whitehall consideration of plans to renationalise Thames Water, with the state taking on most of its £15.6bn debt and lenders losing up to 40% of their money.

[44] In May 2024, Thames' biggest shareholder OMERS issued a "full writedown" of its 31.7% stake in Kemble, signalling its view that the shares were now worthless;[45] another (unnamed) lender sought to offload loans worth up to £600m.

Advised by investment bankers Lazard, Ofwat was trying to make Thames Water attractive to investors, while minimising pressure on consumer bills.

[50] On 11 July 2024, Ofwat put Thames Water into special measures, with a "turnaround oversight regime" subject to "heightened regulatory" scrutiny.

The company had £23bn of assets in urgent need of repair, with supply to 16 million water customers "on a knife-edge", amid concerns over safety, essential IT systems (some dating from the 1980s, now obsolete and prone to cybercrime),[57] and intimidation of staff.

[75] While over 100 investors, bankers, lawyers and other advisers attending the case (racking up huge fees - Thames alone was reported to be paying about £15m a month for the restructuring),[76] the judge criticised Ofwat and the UK Government for not being represented at the hearing.

[82][83] As of 2022[update], Thames Water generated 510 GWh per year, or 24% of its total heat and electricity requirements, using renewable energy from biogas, sewage sludge, wind, and solar.

The incident occurred at its Coppermills Water Treatment Works in Walthamstow, London E17 in April 2010 when an excavator reversed over and killed the worker in a slow sand filter.

[100] In awarding the fine, Judge Francis Sheridan noted the company's "continual failure to report incidents" and "history of non-compliance", saying: "This is a shocking and disgraceful state of affairs.

[104] Thames Water had failed to complete a £1.1 billion programme involving around 108 scheduled upgrades promised in 2018 to enable sewage treatment works to meet legal pollution limits.

An anti-sewage pollution campaign group accused Thames of a "deliberate act to keep it financially afloat" and called for an investigation by regulators.

[105] Thames Water blamed its failure to deliver the schemes on macroeconomic events, including the COVID-19 pandemic and rises in inflation, plus supply chain problems and competing pressures to create more storm tank capacity.

[114] On 9 December 2011, Thames Water was fined £60,000 after releasing sewage sludge into the Foudry Brook killing up to 20,000 fish in a three-mile stretch from Silchester, Hampshire.

Untreated sewage with a high ammonia content was discharged into the Fawley Court ditch and stream that flows into the River Thames at Henley-on-Thames.

One of these incidents involved the company allowing approximately 79 million litres of untreated sewage to escape from a manhole cover into Green Lane Recreation Ground, New Malden, a nearby woodland, and the Hogsmill River.

Without rainwater, discharged sewage is likely to be less diluted, building up toxins and resulting in toxic algae growths that can be fatal to animals and pose a health risk to swimmers.

Dry spilling occurred throughout the summer period when the Thames Water region was declared to be in drought, including on 19 July 2022, the hottest day on record when many people and their pets tried to cool off in rivers.

[124][125] For over 1,000 hours over six weeks in late 2024 and early 2025, Thames Water pumped raw sewage into the River Ver, a chalk stream in Hertfordshire.

[126] In January 2025, Ofwat was investigating why Thames Water failed to complete over 100 environmental schemes (including upgrades to sewage treatment works and reducing wastewater spillages) funded by customers.

[133] Thames Water maintains commercial flocks of sheep on the borders of several of its reservoirs, which are used as the cheapest way to stop large plants growing and damaging the banks.

The concreting of huge amounts of London's green spaces causes substantial rainwater run-off into the drainage and sewerage systems which had been expected to soak into the ground.

As a result, even small amounts of rainfall in certain circumstances can cause London's outdated Victorian sewerage system to fail over, and release untreated sewage mixed with rainwater directly into the River Thames.

The first two stages of the improvements were upgrades to 5 sewage treatment works and construction of the 6.9 km (4.3 mi) Lee Tunnel, formally opened on 28 January 2016.

[152][153] Trinzic launched a campaign calling on the UK government to back 30GW of floating solar installation by 2030, through a mix of policy reform and grant incentives, in November 2023.