A hosepipe ban has been put into place for people living in Hampshire and the Isle of Wight after the region's driest year for a century and highest ever temperatures has caused a drought.
Southern Water says the situation threatens to damage the wildlife habitats of the River Test and River Itchen.
The Temporary Use Ban (TUB) is being introduced for Southern Water customers in Hampshire and the Isle of Wight from Friday 5 August - the first time the restriction has been enforced since 2012.
But Southern Water is insisting there is no direct risk to customers' water supply.
The recent extremely hot weather and reduced rainfall alongside increased demand has left the water levels significantly lower.
A rare red heat warning was issued in July, as temperatures across the south exceeded 40 degrees Celcius.
Dr Alison Hoyle, Director of Risk & Compliance at Southern Water, said: “We haven’t taken this decision lightly and we know the Temporary Use Ban will have an impact on our customers.
"We’re working with the Environment Agency to ensure that we act responsibly to protect our environment. We’re asking everyone in Hampshire and the Isle of Wight to do their bit by supporting these measures and only use the water that they need.
“We’re experiencing one of the driest years on record for over a century and we’ve seen record temperatures. River flows are approximately 25% lower than they should be for July, which is equivalent to losing more than 25 million bathtubs of water. We're asking our customers to help protect our rivers and the habitats that live there by cutting back their water use.
"We believe a Temporary Use Ban is a responsible and vital step to reducing the amount of water being taken from the Rivers Test and Itchen.”
Under the terms of this measure, using hosepipes to water gardens or clean cars will be banned, and ornamental ponds and swimming pools must not be filled.
An Environment Agency spokesman said: “Decisions to introduce temporary bans prohibiting hosepipe-use during periods of extreme dry weather are made by water companies.
“The Environment Agency is working with water companies and other abstractors to protect water resources and ensure they take precautionary action so the needs of water users and the environment are met.
"Water companies are also putting their drought plans in place where needed to maintain water supply, but everyone can play their part to use water wisely.
“Southern Water’s announcement of a temporary-use ban for its customers in Hampshire and the Isle of Wight is one of many measures they and other water companies should consider to reduce unnecessary use of water and protect customer supplies and the environment.
“If you suspect an environmental incident, such as fish struggling in rivers or pollution, please call our 24-hour incident hotline: 0800 80 70 60."