Regulator Ofwat has opened an investigation into South East Water (SEW) after repeated outages across Kent and Sussex.
The investigation will consider whether the company has complied with its licence condition to provide high standards of customer service and support.
Ofwat said it was the first investigation it had launched on customer-focused licence conditions.
SEW has been contacted for a comment.
As of Wednesday night, 10,000 properties continued to have no water supply.
Lynn Parker, Ofwat's senior director for enforcement, said: "The last six weeks have been miserable for businesses and households across Kent and Sussex with repeated supply problems.
"We know that this has had a huge impact on all parts of daily life and hurt businesses, particularly in the run up to the festive period.
"That is why we need to investigate and to determine whether the company has breached its licence condition."
The investigation was started after the prime minister said the situation, which affected 30,000 customers at its height, was "clearly totally unacceptable" and asked Ofwat to review the company's licence.
SEW said some customers might not see supplies return until Friday after issues first began on Saturday in the wake of Storm Goretti and a power cut at a pumping station.
The company said it would be using 26 tankers to pump water directly into its network while working "around the clock" to fix leaks and bursts.
Ofwat already has an open investigation into SEW's supply resilience to determine whether it has failed to develop and maintain an efficient water supply system.