Cassington sewage treatment works, the closest treatment works upstream of popular wild swimming spot Port Meadow, have been referred to the Environment Agency for investigations.

Researchers from the Oxford Rivers Project submitted their data to the EA after finding that, in 2021, the works had spilled when rainfall was as low as 1mm, and sometimes before any rainfall had occurred.

Further analysis indicated that raw sewage was being discharged into the Thames even when the works was operating below full capacity.

It is currently legal for water companies to discharge raw, untreated sewage into rivers in times of ‘exceptionally heavy rainfall’.

Water companies must monitor their overflows and provide data on spills of untreated sewage each year.

It came as a video supplied by Windrush Against Sewage Pollution (WASP) to the BBC appeared to show sewage coming out of a pipe at the works.

The campaign group said they also had an email in which Thames Water confirmed it was also under investigation for "one or more" incidents at Witney.

Thames Water said the EA notified the company last summer that they were conducting an investigation into storm discharges at Witney but nothing new had emerged in the last couple of months.

Some 500 people attended a protest at Port Meadow against sewage dumping hosted by the Clean Our River Thames (CORT) and #EndSewagePollution Mid-Thames groups.

Oxford MP Layla Moran attended the protest and said: "It's absolutely incredible to see so many people, it shows how desperately they want action not just words and we're intent on trying to deliver that for them."

She said a decision is expected imminently from the Government over Oxford City Council's application to have wild swimming spots in Oxford awarded bathing water status.

Richard Aylard, Thames Water’s sustainability director, also attended and explained that the company regards all discharges of untreated sewage as unacceptable and will work with the government, Ofwat and the Environment Agency to accelerate work to stop them being necessary.

Mr Aylard reiterated remarks made previously by the company’s CEO, Sarah Bentley, that the company understands the concerns of river users and that its aim will always be to try and do the right thing for rivers and for the communities who love and value them.

“We have an unprecedented amount of investment directed towards safeguarding our rivers and streams. This includes increasing sewage treatment capacity at our sewage works in the area, including Witney, Fairford and Oxford, and we’re also committed to continuing our industry-leading trial to provide real-time alerts of sewage discharges in 2022,” he said.

Tim Harris, the principal researcher on the Oxford Rivers Project, said: “We first thought something could be wrong when Cassington showed no spills at all on Thames Water’s alerts system since March 2021, despite spilling throughout the year in 2019 and 2020.

“Then, on questioning officers at Thames Water, they stated that the capacity of the works had increased 20 per cent in 2021, an unusually large amount, and despite no investment at the works. This prompted us to look more closely at the data and then submit it to the Environment Agency for investigation.”

Oxford Rivers Project – a programme jointly funded by Thames21, Thames Water, Oxford City Council, and the Rivers Trust – which supports volunteers to collect data on water quality and different river user groups.

A Thames Water spokesperson said: “Cassington sewage treatment works has not discharged untreated sewage since March 2021. The treated effluent has been of a consistently high standard, and an assessment of the watercourse on January 17 found the river area was in good health with very low ammonia levels and high dissolved oxygen.

“We are investigating what is coming out of the 2km long pipe that takes the final treated effluent from the sewage works to the river and have taken further samples as part of our examination. These samples are currently being tested and we are waiting for the results.”

The Environment Agency said they would not hesitate to pursue water companies where there is evidence of non-compliance and take appropriate action – and had concluded seven prosecutions against water and sewerage companies in 2021.

In a statement the EA said: “To date, 1,300 storm overflows and storm tanks at waste water treatment works have been identified as spilling frequently, and prioritised for further scrutiny following our review of data from over 12,000 sites. Our separate, major investigation into possible unauthorised spills at thousands of sewage treatment works is ongoing.”