Water firms in England and Wales have been ordered to return £157.6m to customers due to their poor performance.
Ofwat said the money would come off bills for households and businesses in 2025-26, with the total rebates set to be calculated in December.
Last year, the water regulator ordered firms to repay £114m as part of a similar move.
Ofwat said the results of its annual report on water company performance showed “disappointing results” and that money alone was not enough to address the problems facing the industry.
The regulator also warned that firms were “falling further behind on key targets”, with nine out of 11 suppliers experiencing an increase in “pollution incidents” in 2023.
It comes as water bills in England and Wales are set to rise by an average of 21% over the next five years.
Ofwat’s chief executive David Black said: “This year’s performance report is stark evidence that money alone will not bring the sustained improvements that customers rightly expect.
“It is clear that companies need to change and that has to start with addressing issues of culture and leadership. Too often we hear that weather, third parties or external factors are blamed for shortcomings.”
He added: “Companies must implement actions now to improve performance, be more dynamic, agile and on the front foot of issues. And not wait until the government or regulators tell them to act.”
Ofwat’s report also found that while there had been progress made on leaks, firms had only managed a 6% annual reduction – against a target of 16% by 2025.
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However, four water companies – South East Water, South West Water, Thames Water and Yorkshire Water – were upgraded by the regulator from “lagging behind” to “average”, but it said performance improvements were inconsistent across the sector.
Anglian Water, Welsh Water and Southern Water were all categorised as “lagging behind”.
No firm managed to achieve the regulator’s top rating of “leading”.
Matthew Topham from We Own It, which is campaigning for the nationalisation of the water industry, said: “Today’s action, while a welcome respite from skyrocketing bills, exposes the Catch-22 at the heart of water privatisation.
“Water firms, which desperately need cash to stay afloat, let alone invest to end sewage pollution, will rightly hand back millions they’ve unfairly taken from the public.
“[But] rather than punishing the shareholders behind these failures, our rivers and seas will suffer from even greater underfunding, and the public from future bill hikes in following years, to cover these costs.”
Earlier this summer, the regulator announced it was investigating all wastewater companies due to concerns that some may not be meeting their obligation to minimise pollution.
In August, Ofwat announced that three firms – Northumbrian Water, Thames Water and Yorkshire Water – were facing a combined fine of £168m for a series of failings, including over sewage treatment.
Last year, industry body Water UK apologised on behalf of firms for “not acting quickly enough” on spills.
Years of under-investment by privately-run firms combined with ageing water infrastructure, a growing population and more extreme weather caused by climate change have seen the quality of England’s rivers, lakes and oceans plummet in recent years.
Some water utilities are also creaking under high levels of debt or face criticism over dividends to shareholders and executive bonuses.
Environment Secretary Steve Reed said: “Our waterways should be a source of national pride, but years of pollution and underinvestment have left them in a perilous state.
“The public deserves better. That’s why we are placing water companies under special measures through the Water Bill, which will strengthen regulation including new powers to ban the payment of bonuses for polluting water bosses and bring criminal charges against persistent law breakers.
“We will be carrying out a full review of the water sector to shape further legislation that will fundamentally transform how our entire water system works and clean up our rivers, lakes and seas for good.”