Households in England and Wales are being warned that their water bill could be up to £703 on average, with some regions forced to pay even more.
Every home in England and Wales will see an average 26% water bills increase from April 1, or £123.
It means that the average water bill will be £603 per year, but in some areas the bill will reach £703.
Industry body Water UK has confirmed the bill increases will take effect from April.
Southern Water customers face a staggering 47% increase, Hafren Dyfrdwy and South West Water customers will pay 32% more, Bournemouth Water 32% more, Thames Water 31% more and Yorkshire Water bills will go up 29%.
It means Southern Water customers will pay £703 for an average annual water bill.
The increases are higher than those announced by Ofwat in its new five-year price limits for firms just before Christmas as they include inflation.
The UK is one of only three countries in the world which has a privatised water supply – England and Wales, Chile and some states of the US are the only ones.
The Consumer Council for Water (CCW) said stronger and fairer support was urgently needed to protect struggling households from the largest rise in water bills since the privatisation of the water industry 36 years ago.
Water UK chief executive David Henderson said: “We understand increasing bills is never welcome and, while we urgently need investment in our water and sewage infrastructure, we know that for many this increase will be difficult.
“Water companies will invest a record £20 billion in 2025-26 to support economic growth, build more homes, secure our water supplies and end sewage entering our rivers and seas.”
CCW said customers continued to face a postcode lottery of social tariff schemes, which meant the level of support and who was eligible varied considerably across England and Wales.
CCW chief executive Mike Keil said: “These rises are the largest we’ve seen since privatisation and will heap considerable pressure on millions of customers who are already having to make difficult choices.
“Customers want to see investment in improving services and cleaning up our rivers but that can’t come at an unbearable cost to struggling households.
“Around 2.5 million households are already in debt to their water company and there is a danger that number will grow unless some companies show more ambition around financial support.”
James Wallace, the chief executive of campaign group River Action, said: “We’re being told to celebrate the ‘record investment’ of water companies, but in reality, it is the public that will pay the price for their decades of neglect. Instead of fixing crumbling infrastructure, water companies have saddled themselves with billions in junk debt, leaving us with sewage-choked rivers, and paying extortionate interest rates through bill hikes.
“Communities and customers won’t be fooled by this web of lies. It’s time for broken utilities like Thames Water to be put into Special Administration and refinanced to operate for public benefit not investor return.
“Meanwhile, the Water Commission must end the failed privatisation experiment and reform the broken regulators to ensure a sustainable and resilient water and sewage system for future generations. Rivers do not need economic growth, they enable it.”
