Ofwat's decision to reject Thames Waters' bid to increase prices by 8% is final, but Thames Water could appeal to the Competition Commission.
The company said: "We will review the decision carefully before deciding on our next steps."
Water regulator Ofwat has rejected a bid by Thames Water for an additional price hike for 2014 to 2015 of 8%.
Water regulator Ofwat has turned down Thames Water's application for an additional price increase for 2014 to 2015.
Ofwat says it was for Thames Water to justify the proposed 8% increase was in customers’ interests.
Within the three month timescale for assessing the application, the water regulator found the evidence the company submitted did not justify its proposed £29 additional increase in customers' bills.
Ofwat’s decision means the maximum that Thames can add to customers’ bills for 2014-15 is still 1.4% above inflation, as set in the 2009 price review.
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.
The Lib Dems deputy leader Simon Hughes has called on the water regulator Ofwat to reject a request from Thames Water to raise their prices to pay for the Thames Tideway Tunnel.
A spokesperson for the Liberal Democrats confirmed that the party would present an alternative method of paying for the Thames Tideway Tunnel later this month.
Water providers are coming under pressure from the industry watchdog to make sure drought-stricken parts of Britain are helped by areas with more plentiful supplies.
Regulator Ofwat says the water industry must find creative ways of working to meet challenges posed by a bigger population and climate change. It also wants customers to be given a stronger voice on how water companies deliver services.