A BBC investigation has found that three water companies – Southern Water, Thames Water and Wessex Water, illegally discharged sewage hundreds of times last year on days when it was not raining.

The practice, known as “dry spilling”, is banned because it can lead to higher concentrations of sewage in waterways.

The BBC says that the companies appear to have collectively released sewage in dry spills for 3,500 hours in 2022 – in breach of their permits.

Water UK, the industry body, said the spills “should be investigated”.

The BBC requested the same data from the other water companies in England, which said they could not respond due to being under an Environment Agency (EA) criminal investigation.

Releasing sewage into rivers and seas is allowed in the UK to prevent pipe systems becoming overwhelmed – but it has to have been raining.

Without rainwater the sewage is likely to be less diluted – leading to build-ups of algae which produce toxins “that can be fatal to pets and pose a health risk to swimmers”, says Dr Linda May, a water ecologist at the UK Centre for Ecology and Hydrology.

Discharging in dry conditions is therefore illegal under environmental law.