It has warned customers to expect above-inflation rises until at least the end of the decade, although it cannot yet say by how much.

The region's water customers already pay the highest water bills in Britain at an average of £350.

The likely increases are due to European law which requires more environmental improvements to be carried out.

European legislation is demanding cleaner rivers

In particular, rivers must be cleaned up, more wildlife sites protected and drinking water quality improved.

The region has suffered from high water bills in recent years due to the Clean Sweep programme, introduced to omprove bathing waters.

But with the scheme nearing completion, water customers were hoping to see a reduction in the size of their bills.

However, Ofwat says more environmental enhancements required by Europe will continue to place a heavy burden on bill payers.

Necessary work

Ofwat has just begun its review of charges from 2005, and spokesman Noel Olsen said: "I'm afraid we are very frightened the bills are going to go up.

"There is an enormous amount of work which the Environment Agency and others are saying is necessary.

"At the moment that is all going to have to be charged to water customers."

Keith Richards, of South West Water, said the company saw the need to keep bills as low as possible.

Government advice

But he said the final decision on further increases would not be taken by them.

"That will be made by the independent regulator Ofwat.

"They will be acting on advice from the government as to the pace and timing of environmental improvements," he said.

A two-year consultation process will now begin, with environmentalists, consumer groups and politicians giving their views.

A final decision on the size of water bills will be made towards the end of 2004.