Parts of England are in "crippling drought" with some rivers and groundwater levels lower than in 1976, it has been warned ahead of a Government summit on the issue.
As the South East looks set to follow eastern England into drought, the Environment Department is convening a meeting of water companies, farmers and wildlife groups to discuss potential water shortages.
It comes after the latest Environment Agency briefing said parts of eastern England continue to be drought-afflicted, while swathes of the South East are still affected by dry weather.
They would need well above average rainfall over the next few months for groundwater levels to recover, but the Midlands, Anglian region and South East all had below average rainfall in January.
Ahead of the summit, Thames Water said it was no longer a question of whether there would be a drought in its area, only when and how bad it would be.
Richard Aylard, sustainability director for Thames Water said the Thames Valley and London had received below-average rainfall for 18 months of the last 23 months.
Flows in the River Lee, which goes through Hertfordshire and north east London, are less than a quarter (24 per cent) than the long-term average for the waterway, while the Kennet in Wiltshire has seen flows of less than a third (31 per cent) of average levels.
The Darent, in Kent, is at exceptionally low levels, as is the Wye in Surrey, while a number of other rivers are well below normal levels. The lack of rain has also seen groundwater levels drop significantly, and in some areas they are below the levels seen in the drought year of 1976.
Thames Water is urging people to save water, for example by turning off the tap while cleaning their teeth or taking shorter showers, fixing leaks and only washing full loads of laundry, in the face of potential shortages.
Ahead of the summit, Environment Secretary Caroline Spelman said: "All of our activity is going to be carefully coordinated so that we are able to minimise the effects of unpredictable water availability and support people in doing the right thing in taking sensible measures to save water."