Paul Powlesland, founder of the River Roding Trust, led volunteers in removing litter, silt and vegetation from Alders Brook, a tributary of the River Roding in Essex, earlier this year. The group collected around 200 bags of rubbish, branches and silt during a 10-day operation.

The Environment Agency has since informed Powlesland that it is investigating what it describes as unauthorised works.

In a letter sent to the campaigner, the agency stated: “We consider that unpermitted works have taken place … in contravention of the Environmental Permitting (England and Wales) Regulations 2016.”

It added: “The site is currently under investigation for permitting and waste offences.”

The agency alleges that dredging took place and that waste material was left within the flood plain, activities which may have required an environmental permit. It said further inquiries were being carried out to establish whether any additional offences had been committed and to assess any environmental impact.

Powlesland said he had repeatedly asked the Environment Agency to address conditions on the river but had received no response.

“After decades of ignoring rampant environmental crime on the Roding, the Environment Agency has finally decided to act,” he said.

“But it’s not action against Thames Water for dumping billions of litres of sewage in the Roding, or the waste criminals who have dumped thousands of tonnes of rubbish on its banks, but against the River Roding Trust for … restoring a river without a permit.”