The issue has attracted increasing attention from environmental groups, residents and river users amid ongoing concerns about pollution entering the Thames and its tributaries.
Greg Wilkinson, vice-chair (non-tidal) of the River Thames Society, said residents were right to be alarmed by repeated sewage spills from the treatment works, which he said had a long history of operational problems and pollution incidents.
His comments come against the backdrop of regulatory scrutiny of Thames Water.
In 2024, Ofwat identified Little Marlow among Thames Water’s 13 worst polluting sewage treatment sites as part of a major investigation into the company’s handling of sewage spills and wastewater infrastructure.
The regulator said the company had failed to adequately maintain and operate parts of its network, resulting in excessive discharges into the environment.
Data from Thames Water’s Event Duration Monitoring system has highlighted several incidents in recent years.
In September 2024, untreated sewage was discharged into the Thames from Little Marlow for around 10 hours following heavy rainfall.
Monitoring figures previously reported for the site showed 27 sewage spill events in 2021, lasting a combined total of more than 460 hours.
Campaigners have argued that such incidents point to broader issues with sewer capacity and infrastructure investment in the area.
Mr Wilkinson said independent sampling carried out downstream of Little Marlow had detected high levels of bacteria and viruses, which he said highlighted concerns not only for wildlife but also for people using the river for recreation.
The River Thames Society has called for greater transparency from Thames Water over monitoring data and performance, alongside a funded programme aimed at reducing routine sewage discharges into the Middle Thames.
The organisation is also seeking tougher regulatory enforcement where environmental obligations have been breached and is backing a long-term strategy involving regulators, local authorities and river users to improve the ecological health of the river.
In a statement, Mr Wilkinson said: “Sewage overflows are supposed to be rare, exceptional events, yet for years they have been treated as a routine way to run the system, with the river bearing the cost.”
He added that the society wanted to see the Marlow stretch of the Thames restored to a condition where people could swim, paddle and boat with confidence and where wildlife could thrive.
Concerns about water quality have been voiced by a number of local campaign groups in recent years, while high-profile figures including Olympic rowing champion Sir Steve Redgrave and television naturalist Steve Backshall have also highlighted pollution issues affecting the Thames.
Thames Water was approached for comment but no response had been received at the time of publication.