Ofwat said Thames Water, Britain's biggest supplier and which is struggling to stay afloat financially, could be fined the largest amount at pound104 million.

The regulator's decision comes as Britain's water industry faces huge scrutiny, including from the new Labour government and environmentalists, over the pollution of rivers and other waterways.

"Ofwat has today proposed that three water companies will be fined a total of pound168 million for failing to manage their wastewater treatment works and networks, as part of the first batch of outcomes from its biggest ever investigation," a statement said.

Yorkshire Water could be fined pound47 million and Northumbrian Water pound17 million, it added.

"Ofwat has uncovered a catalogue of failure by Thames Water, Yorkshire Water and Northumbrian Water in how they ran their sewage works and this resulted in excessive spills from storm overflows," said Ofwat chief executive David Black.

"Our investigation has shown how they routinely released sewage into our rivers and seas, rather than ensuring that this only happens in exceptional circumstances as the law intends."