It comes after a string of incidents in which customers were left without water - including some for as long as 23 days.

The firm angered households further in the summer by then introducing a hosepipe ban, which it blamed on more people working from home.

Ofwat said the company was the worst performing in England and Wales for "water supply interruptions", with properties cut off for an average of three hours and two minutes over the past year.

The regulator said customers had been "failed too often" by the firm, which provides drinking water to 2.2 million customers across Surrey, Kent, Sussex, Hampshire and Berkshire.

If the probe finds South East Water guilty of failing to "develop and maintain an efficient water supply system", it could face a fine of up to 10% of its annual revenue, which was £257.5m in the 2022/23 financial year.

The regulator could also force the company to carry out improvement works to its water systems.

Ofwat chief executive David Black said: "Too many customers have been failed too often by South East Water. That's why we are now carrying out a thorough investigation into the company and its service to customers."

He added: "We are clear that water companies must do more to regain public trust and it must start with better service.

"Where this does not happen, we will use all of our powers to ensure the sector delivers better value for both customers and the environment."

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The Consumer Council for Water said it was supporting the investigation into the firm, which it accused of "repeated poor handling of supply interruptions".

Chief executive Mike Keil added there had been "considerable anger and frustration among many people who felt unsupported, compounded by the company's poor communication".

The Drinking Water Inspectorate also announced last month it was considering taking enforcement action "to secure improvements to South East Water's supply system and ability to respond to incidents".

Ofwat said it would be working with the government body during its probe.

It comes after the regulator summoned the firm to an urgent meeting in June to warn that its performance was below what was expected.

A spokesperson for South East Water said the resilience of supplies was a "major focus" and a "significant part" of its 2024 business plan submitted to Ofwat.

They added: "We intend to fully cooperate with Ofwat on this matter."