Customers in the North-East face a possible 37 per cent rise in their bills for water supplies and sewerage.
Northumbrian Water has announced it is asking the regulator Ofwat to approve an inflation-busting increase that it says will take the average household bill from £206 to £286.
The rise, which would be implemented in stages between 2005 and 2010, does not include inflation, currently about three per cent, which would be added to bills automatically.
WaterVoice Northumbria, the region's water watchdog, last night described the proposals as "regrettable but necessary".
Northumbria's increase compares with a national average of about 31 per cent.
The company, which says bills will still probably be among the lowest in the country, plans to seek permission from Ofwat for an unspecified increase in bills from 2004, which could reduce the rise in the following years.
In 2000, the company's 2.6 million customers saw bills fall by 19 per cent and by a further two per cent in 2001.
Managing director John Cuthbert said: "Although we will be investing over £800m in water and sewerage assets in the North-East during the five-year period from 2000, our current price levels are unsustainable."
He said the firm's proposals "will allow us to increase maintenance spending by ten per cent, deliver the required water supply and environmental quality improvements, tackle instances of discoloured drinking water, reduce flooding sewers and will enable us to continue to aim to provide excellent levels of service".
Andrea Cook, chairman of WaterVoice Northumbria, said that customers would not welcome the increase but the previous price reductions were "starting to have an impact on the high levels of service with which Northumbrian Water has always been associated."
Ofwat will decide whether to approve the firm's plan in November 2004 after consultation.
People can give their views to WaterVoice by calling (01325) 464222.
Yorkshire Water is expected to seek an increase below the national average that would see average bills rise from £237 to £270.
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