Price increases announced by South West Water have been criticised by the consumer watchdog.
From 1 April, customers in Devon and Cornwall can expect to pay between 5% and 8% more.
SWW said the average 5.1% increase - part of regulator Ofwat's five-year pricing strategy - would take the average bill to about £517 a year.
The Consumer Council for Water said the rise put pressure on customers who already paid the UK's highest bills.
SWW customers pay about £150 more than the national average.
Bills have increased steadily since water privitisation in 1989 to help pay for Operation Clean Sweep - a £2bn investment to improve beach water quality around the region's coastline, which was completed last year.
Dame Yve Buckland, chair of the consumer council, said water companies were benefiting from inflation.
"We recognise that water companies are facing some additional costs in other areas and that there are 'swings and roundabouts' on this issue," she said.
"However many water companies and their shareholders will benefit from higher inflation."
She said debt was rising and many customers could not afford their water bills.
"The bill announcements today will particularly add pressure to some customers in the south west of England, who already pay the highest bills."
SWW said it would spend £159m this year improving facilities.
According to the consumer council, an estimated three out of 10 people are in "water poverty" - with bills costing more than 3% of their income.
SWW said it recognised there was a problem and had taken action to address it.
"We do know some customers do struggle to pay their bills which is why we will also be offering free water-saving packs for the first time this year," a spokesman said.
"For those most in need, we will bring in a new £1m FreshStart scheme in partnership with local Citizen Advice Bureaux and also expand our WaterCare programme to include free home energy audits."
As well as upgrading water treatment plants and replacing crumbling sewers, SWW said in 2011-12 it would also work to restore 500 hectares of wetlands on Dartmoor and Exmoor, control future water treatment costs and open a new reservoir near Bodmin Moor in Cornwall.
SWW has customers throughout Cornwall and Devon, as well as parts of Dorset and Somerset.