The Environment Agency has criticised water companies over their “unacceptable” performance on sewage spills last year.
The regulator said five out of nine companies in England were rated as “requiring improvement” in its annual report released on Tuesday.
This included Thames Water, the UK’s biggest water supplier, which has been under fire in recent months over pollution and financial woes.
It comes as the sector faces a national crisis after years of under-investment by the privately-run firms combined with ageing water infrastructure a growing population and wetter conditions caused by climate change.
The Environment Agency said there were 47 serious pollution incidents last year – up from 44 in 2022.
Four companies – Thames Water, Anglian Water, Southern Water and Yorkshire Water – caused 90% of those incidents, it added.
Alan Lovell, chairman of the Environment Agency, said: “This is unacceptable.”
“My over-riding sense is one of frustration and disappointment,” he added. “The results we see are, yet again, simply not good enough.”
The annual report said there was a “small improvement” in environmental protection among some firms and that Severn Trent, Wessex Water and United Utilities performed well.
But the regulator said the majority are not getting the basics right, such as minimising pollution incidents and complying with permits.
Mr Lovell said the culture within some firms “can also perpetuate poor practices” and warned that the regulator will not take “bad weather” as an excuse for poor environmental performance.
He said: “Some simply do not understand the root cause of their problems and incidents are not reported in a timely manner – this is vital for improving transparency and trust across this sector.
“We know that with a changing climate, the country is going to experience more extreme weather patterns, increased rainfall and more rapid drought.
“Water companies must ensure that assets are resilient enough to withstand these challenges.”
For its part, the Environment Agency said it would introduce a tougher regulatory approach.
It outlined plans for recruiting 500 additional staff, investing in digital systems, increasing enforcement activity and quadrupling the number of water company inspections to 4,000 before April next year.
Environment Secretary Steve Reed called the findings “shocking”.
“For too long, water companies have pumped record levels of sewage into our rivers, lakes and seas. This Government will never let this happen again.”
The new Government has announced plans to crack down on water firms, including tougher fines for pollution incidents and ensuring money is invested into infrastructure rather than paid out in bonuses.
“Change will take time, and we will outline further legislation to fundamentally transform our water industry and restore our rivers, lakes and seas to good health,” Mr Reed added.
A spokesperson for Water UK, the sector’s trade association, said: “While today’s results show overall water company environmental performance has improved, it is clear there is more to be done.
“The performance of some companies, as they acknowledge, is not improving fast enough and pollution incidents remain too high.
“Water companies have proposed investing a record £105 billion to secure our water supply in the future and stop sewage entering our rivers and seas. Ofwat needs to approve these plans in full as any less will put critical improvements at risk.”
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