Carolyn Roberts, a professor and water and environment consultant, revealed she had lodged an initial claim against Severn Trent Water and planned further legal actions against five other firms on behalf of more than 20 million customers.
Roberts, who is represented by the well-known British law firm Leigh Day, estimates the companies could face compensation bills of over 800 million pounds ($1 billion) if the cases are successful.
The first claim, filed last week against Severn Trent on behalf of eight million people, is estimated to be worth more than 330 million pounds.
It comes amid a long-running scandal over privatised water firms pumping raw sewage into waterways, provoking widespread public anger and promises of increased regulatory scrutiny from the government.
Ministers announced last month that companies and individuals polluting Britain's rivers and other ecosystems will be liable for unlimited fines.
"Like many others across the country, I have viewed with horror the escalating number of stories in the media regarding the volume of sewage discharged into our waterways and on to our beaches," Roberts said in a statement.
"It appears that because of the serial and serious underreporting at the heart of these claims, water companies have been avoiding being penalised by Ofwat," she added, referring to the sector's regulator in the UK.
"I believe this has resulted in consumers being unfairly overcharged for sewage services."
Industry body Water UK said the accusations are "entirely without merit" and that 99 percent of sewage works are legally compliant.
However, Roberts and Leigh Day said they intend to bring similar "collective actions" against Thames Water, United Utilities, Anglian Water, Yorkshire Water and Northumbrian Water.
They urged the companies' millions of customers to visit a website created to assess their eligibility for compensation.
"These companies have allegedly been misleading their regulators by underreporting the number of pollution incidents, being discharges of wastewater from a company sewerage asset adversely affecting the water environment and resulting in higher customer bills," a statement on it read.
Last month, a UK court fined Thames Water, the nation's biggest supplier, 3.3 million pounds for polluting rivers.
The fine came shortly after the UK's privatised water companies pledged to make massive investments to avoid repeats of the contamination.
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