Water demand has shot through the roof and water companies are struggling to keep up.

Southern Water's increase is equivalent to supplying an extra 850,000 people in its region every day.

The company, based in Worthing, said it had enough water resources but during peak-demand times, particularly when people arrived home from work, it was proving impossible to pump supplies through water mains to taps quickly enough.

The amount of water being pumped from the company's key Itchingfield Reservoir in north Sussex had risen to 133 litres every second during the early evening, compared with 34 litres every second at the beginning of the day.

Across Kent, Sussex, Hampshire and the Isle of Wight, Southern Water was now supplying in excess of 737 million litres a day, compared with the normal figure of 600 million litres.

The company said garden watering was key to demand levels, with sprinklers using as much water in an hour as a family of four did in two days.

Distribution manager Jon Cooke said: "There is a finite limit to the amount of water that can physically be pumped through our mains at any one time.

"If the peak demand for water continues to rise, there comes a point when we can't pump the water quickly enough, no matter how much we have in reserve.

"Therefore, we are urging people to use water wisely.

"We don't want anyone to stop watering their gardens but please do so sensibly and not waste supplies."

Southern Water spokeswoman Rebecca Burgess said: "Watering gardens during the day and the evening sunshine is a complete waste of water.

"Gardens will benefit only if watered while the sun is down."