Britain’s water industry regulator has said it plans to block Thames Water’s proposal to increase customer bills by 8% next year.

Ofwat has published a draft decision that would disallow the water company’s request for the interim rise, which would add an extra £29 to the annual average household bill – currently at £354 a year.

Water firms need permission from Ofwat to raise prices beyond limits set by the regulator for five-year periods. Ofwat said Thames Water was permitted to increase prices by 1.4% above inflation for 2014/15.

Sonia Brown, Ofwat’s Chief Regulation Officer said: “We said we would challenge Thames Water’s request. We have looked at the details and do not believe the current evidence justifies an increase in bills.”

The draft decision will be followed by a short period of technical consultation to allow for any new evidence before announcing the final decision next month.

A spokesperson from Thames Water said: “We will review Ofwat’s draft proposals and submit our response in due course.”