The water watchdog has aired "concern" as 25,000 properties remain affected by outages across Kent and Sussex - a day after a major incident was declared in the two counties.
The latest supply failure is being blamed on Storm Goretti causing power cuts and burst pipes.
Many people are into a fourth day without water, with the latest outages coming overnight between Friday and Saturday last week.
South East Water (SEW) has since said water supplies to Loose in Maidstone, Blean near Canterbury, Headcorn, West Kingsdown and parts of Tunbridge Wells had been restored on Tuesday.
After issuing another apology, it added that drinking water to 16,500 properties in East Grinstead should be restored on Tuesday afternoon.
Ofwat, the water services regulation authority, said it would "review all the evidence" before deciding whether SEW has met its legal obligations to customer care.
Read more from Sky News:
'Major incident' declared in Kent as thousands without water
Ex-soldier jailed for Livingston Police Station petrol bomb attack
The areas affected by water supply issues include: Maidstone, Sevenoaks, West Sussex and surrounding areas, Tunbridge Wells, Canterbury, and Whitstable.
Linden Kemkaran, leader of Kent County Council (KCC), declared a major incident across the county on Monday morning, saying more households had been impacted in the previous 24 hours.
Several schools in Kent and Sussex were forced to close on the first day of the week.
It comes after 24,000 properties in Tunbridge Wells, Kent, spent days without any running water, and almost two weeks without drinkable water last month.
SEW apologises to customers
SEW said Storm Goretti, which hit the UK last week, affected its ability to "treat the raw water at the normal rate at our main Sussex water treatment works".
It said this, coupled with the outbreak of burst water mains due to freezing conditions across Sussex, means its drinking water storage levels are running low.
SEW incident manager Matthew Dean told Sky News: "We're very sorry to our customers across Kent and Sussex who continue to experience issues with their drinking water supply, including no water, intermittent supply interruptions or low pressure.
"We are using 26 tankers to pump water directly into our network to increase the amount of water available in the affected areas and our leak repair teams are working around the clock to fix the leaks and bursts across Kent and Sussex, with extra resources available to help carry out repairs."
He said SEW's customer care team was "delivering bottled water to the customers on our Priority Services Register who are most in need".
"We are also supporting hospitals with tankers and providing bottled water for care homes, schools, medical care providers and to support livestock," Mr Dean added.
Bottled water handed out to residents
SEW said it has carried out 5,700 deliveries of bottled water over the weekend.
Bottled water stations remain in place in Tunbridge Wells, East Grinstead and Maidstone, and location details can be found on SEW's website.
Be the first to get Breaking News
Install the Sky News app for free
An Ofwat spokesperson said on Tuesday: "We are concerned that residents in Kent and Sussex are without water again, and are working closely with the Drinking Water Inspectorate, which is the lead regulator for this latest supply interruption, to ensure that regulation and enforcement is aligned.
"Ofwat already has an active investigation into South East Water related to its supply resilience, and we have met with the company to discuss these latest incidents as part of that investigation.
"We will review all of the evidence before taking a decision on what further action may be required into whether the company has met its legal obligations set out in its licence relating to customer care, including with further potential enforcement action."