Water companies have been consistently under-reporting the number of times they cause pollution incidents and overcharging customers as a result, according to legal firm Leigh Day, which is bringing a collective case against Severn Trent Water, Thames Water, United Utilities, Anglian Water, Yorkshire Water and Northumbrian Water. 

Professor Carolyn Roberts, an environmental and water consultant represented by Leigh Day, is bringing the claims on behalf of more than 20 million household customers. The first claim alone, against Severn Trent Water, is brought on behalf of 8 million customers with an estimated value of more than £330m. If the six claims are successful, they are expected to lead to compensation payments of over £800m.

In May, Water UK, which represents England’s nine water and sewage companies, promised to undertake a huge modernisation of England’s sewer system following concerns over repeated sewage spills.

Roberts alleges that the six...