Three water businesses owned by France's Veolia and serving more than 3.5 million people, including customers in Dover and Woking, were sold in a deal worth £1.2 billion.
Veolia Water's Central, South East and East businesses are to be sold to an infrastructure fund managed by Prudential-owned M&G and Morgan Stanley.
The three water companies have a combined annual turnover of £274 million and collectively employ around 1,250 people.
Veolia Water, which will retain a 10% stake in the water supply businesses, said the move would allow it to focus on its work as a leading provider of outsourced services.
More than 1,000 people work for Veolia Water Outsourcing, helping to generate turnover in excess of £400 million a year.