WELSH Water is one of 11 firms that have been ordered to reduce the price of their bills because they missed targets set by the industry regulator.
Most water companies in the UK are being hit by financial penalties from Ofwat.
It means overall nearly £150 million is to be taken off customers' bills in the next financial year.
Companies missed targets on areas such as water supply interruptions, pollution incidents, and internal sewer flooding, Ofwat said.
Here, Welsh Water will have to cut bills by a total of £8 million next year.
"When it comes to delivering for their customers, too many water companies are falling short, and we are requiring them to return around £150m to their customers," said Ofwat chief executive David Black.
"We expect companies to improve their performance every year; where they fail to do so, we will hold them to account."
He added: "All water companies need to earn back the trust of customers and the public and we will continue to challenge the sector to improve."
The financial penalties announced this week are based on whether companies have hit their targets on a range of issues.
Poorer performing companies face a financial hit because of missed targets on water treatment works compliance, pollution incidents and internal sewer flooding across 2021/22 and will have to reduce customer bills accordingly.
The company facing the biggest penalty is Thames Water, which will be forced to cut its customers' bills by £51 million next year.
Not all water companies have significantly missed their targets. Better performers such as Severn Trent Water have exceeded their targets in areas like biodiversity.
It means Ofwat will allow the company to charge more from customers next year, to the tune of £62.9 million.
These figures announced this week are draft decisions, which are now out for consultation and can be viewed on the Ofwat website. The consultation closes on October 21.
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