Bills for customers of South West Water (SWW) could go up by as much as 8.1% this year, the BBC has learnt.
The figure comes from regulator Ofwat's five-year pricing strategy, due to be published on Friday.
Business groups and organisations representing older and retired people said the increase would squeeze already hard-pressed incomes.
SWW said it would confirm investment and charges plans on Friday. It added that most would not see an 8.1% rise.
Ofwat is to allow SWW to increase prices at the inflation rate for last November, 4.7%.
The company can then increase prices by another 3.4% on top of that to invest in new technology and infrastructure, bring the total up to 8.1%.
SWW has customers throughout Cornwall and Devon, as well as parts of Dorset and Somerset.
Customers currently pay among the highest charges in Britain - with annual bills up to £150 more than the national average of £340.
Martin Rogers, from Age UK in Exeter, said that although SWW had made improvements in its business and services, any rises would still result in problems for anyone struggling to make ends meet.
He said: "This kind of rise is going to be another nail in the coffin. It's going to be really difficult.
"It's about people looking at their finances and wondering if they can maximise them, or is it worth having a benefits check; is it worth having a water meter?"
SWW said that average billing increase was likely to be lower than 8.1%, depending on a customer's tariff and whether they were on a water meter.
It added that it offered to people who struggled to pay their bills.