A legal action group is warning there may be an information blackout on sewage discharges by water companies for years, after the Environment Agency announced an inquiry into more than 2,000 sites.

Fish Legal said that in the past, the EA has refused to disclose any information once an investigation is launched. The group said the systemic abuse of the permit conditions under which sewage treatment works have to operate had been brought to light by the public, often using environmental information requests.

If that access to information was shut down it would make it impossible for the public to engage with the process of assessing harm caused by these spills.

The EA announced on Friday it was investigating water companies which had admitted they may have illegally discharged raw sewage from their treatment plants. Their admissions came during discussions with the agency about the installation of flow monitors on treatment plants that could show up any illegal discharges.

The investigation into potentially illegal dumping of sewage is huge in its scale and involves more than 2,000 treatment plants.

But given the length of time it has taken in the past for the EA to investigate and take enforcement action, in some cases many years, Fish Legal fears there will be a total information blackout.

Penny Gane, head of legal practice at Fish Legal, said: “We are seeking assurances that a veil of secrecy is not about to descend on the operation and impact of storm overflows.”

Fish Legal, which was previously known as the Anglers’ Conservation Association, takes up cases on behalf of its members involving pollution, abstraction and poaching. It has 24 active cases relating to sewage pollution by water companies.

In a letter to Sir James Bevan, the chief executive of the EA, Gane called for transparency and openness as the agency ran its investigation.

“We have serious concerns that the agency will effectively shut down all information-sharing about the operations of these works whilst its investigation is ongoing,” she said.

The EA held a briefing on Friday with NGOs and environmental groups, including Fish Legal, after its announcement of the latest investigation.

Gane was not reassured by the meeting. In her letter she said: “We have, in the past, battled for disclosure of information relating to pollution incidents.”

Gane highlighted the length of time it had taken for the EA to investigate and prosecute Southern Water over the illegal dumping of billions of litres of untreated sewage. The investigation into offences that took place between January 2010 to December 2015 only ended in July 2021 when the company was fined GBP90m.

“We therefore consider there to be a very real prospect that the agency will impose an information blackout relating to storm overflows for years whilst its investigation is ongoing,” said Gane.

“[I]t will make it impossible for the public to effectively engage with the process of assessing harm caused by these discharges.”

Gane called for Bevan to disclose which treatment plants were under investigation.

An Environment Agency spokesperson said: “This week, the Environment Agency and Ofwat have launched a major investigation into sewage treatment works. Investigations are beginning and, as would be expected, we must be guided by legal advice on what we can say and when in order not to prejudice any future legal proceedings. Data related to over 12,000 storm overflows is already online for everyone to see.”