Six British water companies are accused of under-reporting sewage discharges and overcharging millions of customers up to £1.5 billion ($2 billion), according to lawyers involved in a landmark lawsuit presented to a London tribunal on Monday.

The utilities, including Britain's largest water provider Thames Water, allegedly misled industry regulator Ofwat about the number of pollution incidents, enabling them to charge higher prices to customers. The six companies – which also include Anglian Water, Northumbrian Water, Severn Trent, United Utilities, and Yorkshire Water – argue that the case should be dismissed.

This lawsuit, the first environmental competition law action in the United Kingdom, comes amid growing public outrage over the amount of sewage being pumped into Britain's waterways, prompting the government to promise stricter oversight. Environmental and water consultant Carolyn Roberts is spearheading the case, which alleges that the six water companies 'significantly and/or systemically under-reported' the number of pollution incidents to Ofwat.

Julian Gregory, a lawyer representing Roberts, informed the Competition Appeal Tribunal that Thames Water may have failed to report over 6,000 discharges. Roberts' lawyers estimate the case against Thames Water, which is heavily indebted and attempting to raise funds to stabilize its finances, to be worth up to around £200 million.

Gregory argued that the six cases should be certified to proceed towards a trial, a preliminary step in the case. However, the water companies contend that the case is precluded by English law governing the water industry and should be dismissed. Industry body Water UK stated: 'This highly speculative claim is entirely without merit. The regulator has confirmed that over 99% of sewage works comply with their legal requirements.'

The hearing is set to conclude on Thursday, with a decision on whether the case can proceed expected at a later date.