The government and Ofwat will make the final decision on prices

SWW wants to increase the average household bill by more than 20% above inflation over a five-year period, which it has announced in its draft business plan for 2005 to 2010.

Under the plan, the average bill would rise by �73 to �407.

The company says it wants charges to increase to pay for improvements in the supply network because some water mains are old and unreliable.

Tolerance to high bills is overstretched already Dr Noel Olsen, WaterVoice South West

WaterVoice says many customers are already struggling to pay their water bill now and there is a lot of hostility to higher prices.

Dr Noel Olsen, the chairman of WaterVoice South West, said: "The biggest issue for customers of South West Water is affordability. We cannot emphasise too strongly that many customers struggle to pay their water bill now, let alone a higher one.

"Customers are clearly hostile to further price rises. While South West Water is proposing smaller rises than those sought by environmental groups, tolerance to high bills is overstretched already."

The watchdog says South West Water has not provided enough evidence to justify some of its proposals, and it is suggesting that some of the investment in mains could be deferred.

SWW said its business plan is a realistic assessment of future need and that it did not want to have to raise prices.

It said: "It builds on the significant improvements to customer service and the environment over the past 10 years.

"We consider it critical that there is no deterioration in the quality of the environmental improvements which our customers have had to fund since 1989 and which have been the prime driver for price increases over the period."

The government and Ofwat will make the final decision on what South West Water can charge.