Thames Water came under renewed fire yesterday for failing to meet customer service standards.
Ofwat plans to fine the country's biggest water company for the breaches.
The announcement comes weeks after the regulator ordered Thames Water to pay £150 million for missing leakage targets.
The latest breach relates to the company's failure to meet customer service standards and compensate those affected.
Customer standards relate to areas such as answering enquiries, dealing with complaints and keeping people informed.
An Ofwat spokeswoman said it was not yet known how much money Thames owed customers in compensation.
Thames Water told the regulator about breaches in January. It has since pledged to reimburse customers and step up customer service.
Ofwat and Thames Water have ordered an independent investigation.
The regulator will announce the level of fine and whether further action is needed once the investigation is complete.
The company's customer services director Phil Scholes said: "We have fully cooperated with this investigation so we can establish how these errors were made and make sure that they are not repeated in the future."
Compensation payments for breaches of service standards are usually around £10 to £20 per household, Ofwat said.
Under new powers introduced on April 1, Ofwat can fine water companies up to ten per cent of turnover.
Earlier this year the regulator announced plans to fine both Southern Water and Severn Trent Water for customer service failures.
* A water company which misreported its customer service performance targets to Ofwat confirmed yesterday it will be investigated by the Govern-ment's Serious Fraud Office. Southern Water has paid out more than £150,000 to 6,000 customers because they never received compensation.