Campaigners and opposition MPs have called for action on sewage spills, following a BBC News investigation.
It suggests three major water companies illegally discharged sewage hundreds of times in 2022 on dry days.
The practice, known as “dry spilling”, is banned because it can lead to higher concentrations of sewage in waterways.
The Environment Agency (EA) said it was currently conducting its largest criminal investigation into “non-compliance” by water companies.
Water companies are allowed to release sewage after it has rained, to prevent it overwhelming the system and backing up into people’s homes.