Britain's water industry regulator Ofwat said that an average eight percent hike in water bills planned by Thames Water was not justified.
"We said we would challenge Thames Water's request," Ofwat's chief regulation officer, Sonia Brown, said. "We have looked at the details and do not believe the current evidence justifies an increase in bills."
Thames Water, which has 14 million customers in London and the southeast of England, submitted a request in August to increase prices above the limits agreed in 2009.
Ofwat said it would take a final decision on the increase next month after a short period of technical consultation, which allows for the submission of new evidence.