Ofwat said the enforcement case against the company, currently fighting to secure its financial future amid a £19bn debt pile, aimed to determine whether it had breached its legal obligations.
The watchdog said that Thames itself had been in contact to alert it and the Environment Agency that it will be unlikely to deliver more than 100 of 812 improvements it had promised by a 31 March deadline.
Thames Water is being investigated over late delivery of environmental improvement schemes, the industry regulator has announced.
