Rainfall has been below average for four months

The company supplies about 650,000 people across east Surrey, West Sussex, west Kent and south London. Groundwater makes up 85% of its water supply.

A conference on Friday heard latest rainfall and resource figures, and the implications for future water use.

Rainfall in the region has been below average for the past four months.

The company said 15% of its supply comes from one reservoir at Bough Beech, west Kent.

Climate change warning

On Friday, the Environment Agency said some of the greatest extremes of weather in Britain had occurred in the last decade.

The agency said today's 10-year-olds had lived through six of the UK's hottest years on record.

An Environment Agency report published on Friday, The Climate is Changing: Time to Get Ready, warned that climate change was now "the biggest threat to our future".

Chief executive Barbara Young said: "It is clear that climate change has been triggered by increases in greenhouse gases, in particular those released from our burning of fossil fuels.

"Already, in the first years of this century, floods, storms and drought have shown how vulnerable the UK is to the weather."

Conservation pleas

This month, Southern Water issued a drought warning to its customers in Kent and Sussex, saying the Weir Wood reservoir, near East Grinstead, West Sussex, was only 57% full when it should be 90% full.

The company has asked people in East Grinstead and nearby Crawley to conserve water in the coming months to prevent a shortage this summer.

The water company said heavy snowfall had not helped the situation as 1ft of snow is equivalent to one inch of rain.

Weir Wood reservoir is the only reservoir in the region to rely on rainfall, whereas other places, such as Bewl Water, can have water pumped to it.