Five UK water companies have appeared in court proceedings for the first time over allegations of under-reporting pollution incidents and over-charging customers as a result.

Severn Trent Water, United Utilities Water, Yorkshire Water, Northumbrian Water and Anglian Water are alleged to have failed to reveal the true scale of sewage discharges and spills made into UK waterways.

They also are alleged to have breached competition law because any under-reporting could have affected the price they could charge customers.

The claims are being brought by Professor Carolyn Roberts, a water resource management specialist, on behalf of an estimated 20 million affected customers.

Law firm Leigh Day, which is representing Prof Roberts, said households could receive a share of up to £800 million in compensation if successful.

Legal representatives for the five firms appeared at the Competition Appeal Tribunal in London for a preliminary hearing in London on Tuesday.

Prof Roberts and the respondents already agreed the claims against all five firms should be heard together.

Industry regulator Ofwat, which is currently carrying out its own investigations into regulatory breaches, also attended the hearing on Tuesday amid questions over how the claims being brought by Prof Roberts might relate or interact with its own probes.

Mr Justice Marcus Smith set the date for a hearing to decide whether the claims are suitable to proceed as opt-out collective claims – where all affected customers are included in the claim unless they request to be removed – and approve Prof Roberts as the class representative.

This “certification” hearing will take place over a week from September 23.

But Mr Justice Marcus Smith also set the date for a second hearing from January 13 2025, which can cover any additional issues due to the complex questions around Ofwat’s own investigations and powers under the Competition Act.

The judge said: “The process is clearly not going to be as straightforward as the usual certification process.”

He later added: “We may not neatly resolve everything in the week allocated as ‘week one’. We very much anticipate and hope to do so but these are knotty issues and we are at an early stage, so we want to have that adverse contingency covered off.”

If certified, the claims can proceed to a full trial, which Leigh Day estimates will take at least two to three years.

The case comes as the UK’s first environmental collective actions.

Claims are also being brought against Thames Water, but the firm was not involved in the hearing on Tuesday since those proceedings are not as not as far along compared to the other five firms.

Mr Justice Marcus Smith said nothing in his rulings on Tuesday will affect Thames Water’s position, although the firm may join the process in the coming days and weeks.