Officials at the agency are currently reviewing the Grimston site, which is located east of King's Lynn about 600 metres upstream from Derby Fen - a Site of Special Scientific Interest.

It followed campaigners who are calling for better protection of the Gaywood River, which the storm overflow releases into, conducting tests which they say found high levels of pollutants at the nature site.

An aerial photo of the Grimston sewage works (Image: Gaywood River Revival)

Tests conducted last month by Gaywood River Revival found high levels of E.Coli at Derby Fen, a bacteria which comes from human and animal waste.

An Environment Agency spokesman said: "Protecting water quality and the environment is a priority.

"It is completely unacceptable that so much sewage is still entering our waterways and it cannot continue.

"While we welcome the progress that has been made to reduce spills, we share the public's anger that it’s not enough.   “This year, we have completed over 1,600 compliance inspections of Anglian Water wastewater assets, including targeted inspections of high‑spilling storm overflows.

"We are currently reviewing the performance of the Grimston overflow in line with our regulatory responsibilities."

Derby Fen is a site of special scientific interest (Image: Gaywood River Revival)

The EA added that last month, a blockage caused a "premature discharge" into the river and that Anglian Water has raised concerns about infiltration in the Grimston area.

The water firm has invested heavily in making improvements in the area, which is considered one of the worst spots for sewage issues.

This has included a major relining project for the sewers but there continues to be a problem with groundwater and surface water infiltrating the pipes.

The Environment Agency says it will investigate if any permit conditions have been breached or environmental harm has occurred.

Since 2010, there have been two "substantiated incidents" at the Grimston site.

Engineers carry out relining work in Grimston (Image: Anglian Water)

Mark Dye, spokesman for Gaywood River Revival, fears incidents may have been missed due to limited site inspections.

He said: "If the EA has carried out just one routine site inspection at Grimston since September 2025, it’s clear that a catalogue of documented illegal sewage spills have been missed not only here, but also elsewhere the Gaywood continues to suffer from serious pollution incidents.

"The fact that the EA suggests there have been just two ‘substantiated incidents’ at the Grimston Sewage Treatment Works in 16 years suggests that either they have been looking the other way, or that something has gone terribly wrong in terms of environmental care and its regulatory responsibilities.”

An Anglian Water spokeswoman has said it is looking closely at the data to understand whether there's further work to do to reduce infiltration.

It aims to install river quality monitors by 2030, which will help monitor the quality of watercourses and "enable us to engage with other parties to help address other sources of pollution or poor water quality".

An EA spokesman added: “We encourage anyone who suspects environmental pollution to report it to the Environment Agency’s 24-hour incident hotline on 0800 807060."

© Eastern Daily Press