The water regulator has taken a public swipe at some of the sector’s largest suppliers by warning customers that it cannot take what water companies say at face value.

The startling broadside pours doubt on whether suppliers can be trusted to give clear and transparent information about their finances and their performance against regulatory targets.

Ofwat said the worst offenders - across a range of eight measures - were Thames Water, Dee Valley, Southern Water and Bristol Water.

Aileen Armstrong, a senior director at Ofwat, said: “We want to be able to trust what we get from our water company – be that the water out of the tap or what they tell us. Unfortunately, on the second of those, our checks suggest we might not be able to take everything at face value.

“These businesses provide essential public services and they need to assure customers they are doing the right things in the right way. If they are to gain and keep the trust of customers, they need to have high-quality checks on their information and present it fairly, clearly and completely.”

The damning indictment was made following the regulator’s review of Britain’s 17 water companies against eight measures. Only three categories revealed serious concerns about the four companies.