Discharges of more than two billion litres of raw sewage from Mogden treatment works into surrounding waterways are being investigated by the Environment Agency.

The equivalent of 800 Olympic-sized swimming pools of raw sewage was released across 48 hours in October 2020.

These are among a series of spills linked to the Isleworth plant which have blighted local rivers and wildlife, alarming residents, local councillors and MPs.

The problems associated with Mogden, which is run by Thames Water, were recently detailed in a report published by MPs on the Commons Environmental Audit Committee (EAC) on the water quality of the UK's rivers.

And it has now emerged that the regulator, the Environment Agency, has launched an investigation into sewage spills from Mogden.

An Agency spokesman said there has been significant investment at sewage works to improve water quality in the Thames.

But, it said: "Our routine compliance monitoring activities have flagged the need for further investigations into operations at the Mogden site.

"Water companies are required to provide a record of sewage discharged into our waterways and have a legal duty to avoid pollution.

"They must act quickly to reduce any damage that happens as a result of their activities."

Mogden, which is the UK's third largest sewage treatment works, poured one billion litres of contaminated water into the Thames on October 3 and the same amount was dumped on October 4, 2020.

Nub News revealed last year, how Thames Water was condemned for delays in cleaning up a devastating storm water sewage spill from Mogden into the Duke of Northumberland's River and on to the Thames.

The Twickenham MP, Munira Wilson, recently raised her concerns about Mogden on a visit to the plant, which receives sewage from 2.17m people across west London.

The site treats 12,314 litres of sewage per second but will need to invest £100m over the next three years to support population growth.

The MP said: "Discharge into the river should be an extremely rare occurrence and I urged Thames Water to do everything in their power to reduce the number and scale of these incidents."

The Thames Water chief executive, Sarah Bentley, said that problems in October 2020 involved two of the wettest days on record.

Mogden's eight storm tanks were unable to cope with the volume of rain, causing sewage to be dumped into the Thames.

Ms Bentley said: "We would have needed either another treatment works the same size as Mogden treating another 1 billion litres or we would have needed 150 more storm tanks (to cope with the rainfall)."

Sewage overflows are legal when sewers are likely to become overwhelmed.

Releasing the sewage into rivers prevents it from backing up into homes and spilling out of toilets and bath plugholes.

Residents living close to Mogden Sewage Treatment Works have complained about the works for several years.

Councillor Salman Shaheen, who represents the ward of Isleworth for Hounslow Council, said: "Mogden needs to upgrade their facilities. They must increase its storm tank capacity to prevent spillages into the Thames. It can't be allowed to continue at this level."

Thames Water said: "We regard 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.

"We have an unprecedented amount of investment directed towards safeguarding our rivers and streams.

"Between 2020 and 2025 we are spending £1.25 billion on maintaining and improving our operational sites.

"We continue to upgrade Mogden sewage works to help meet growing demand and we have £22 million of investment planned to improve the site's performance and power generation capabilities.

"Our aim will always be to try and do the right thing for our rivers and for the communities who love and value them. We have a long way to go – and we certainly can't do it on our own – but the ambition is clear."