Customer complaints against Severn Trent Water have risen dramatically, according to the latest figures.
Severn Trent Water made £270m profit in the past year
There were 1,100 complaints about the Birmingham-based firm in the six months until March 2006 - a rise of 55.6% on the previous year.
It comes after the regulatory body Ofwat said it intended to fine the firm over its poor customer service.
There was a 41% rise in complaints at United Utilities but 4.5% fewer complaints about all other water firms.
Billing issues were the single biggest source of complaints to the Consumer Council for Water, which released the figures on Thursday.
Dame Yve Buckland, the council's chair, said: "From our research we know that consumer trust is getting stretched.
"They expect to see service improvements and reliable supply alongside returns to shareholders."
Last month Severn Trent announced £270m profits, a rise of 18% in the same year customers were asked to pay 15% higher prices.
The firm, which supplies customers across the Midlands, also saw leakages rise by 3% in the past year.