Utilities regulator Ofwat has warned Anglian Water and three other water companies that it has "substantial concerns" over their business plans for the next five years.
Anglian Water, which supplies water to 4.3 million people across the East of England, is one of the companies which have received letters from the watchdog calling on them to review their cost proposals.
Ofwat has also raised concerns over the proposed business plans of Thames Water, Yorkshire Water and SES Water.
The regulator said Anglian, Thames and Yorkshire have asked to increase the amount they spend on wholesale water supply.
In December 2017, Ofwat told suppliers it expects them to make a step change in efficiency by 2025, the end of the next regulatory period.
It called on companies to deliver better services for customers, and protect and improve the environment, while at the same time keeping bills low.
Following scrutiny of the companies' revised proposals for the 2020-2025 regulatory period, Ofwat said it does not believe the cost plans are suitably efficient.
David Black, senior director at Ofwat, said: "We said we would scrutinise each and every plan in detail to ensure they are robust enough to deliver a high quality, affordable and resilient service to customers in the next five years and beyond.
An Anglian Water spokesperson told ITV News Anglia: “As the company serving the driest and one of the fastest growing parts of the country, we’re confident our plan is the right one to ensure a sustainable future for our region."