The South East is now in a state of drought, according to the Department for Environment, Food and Rural Affairs.
The region joins parts of eastern England which have been in the grip of drought since last summer.
Levels in some reservoirs, rivers and groundwater aquifers in the South East are well below normal levels after two exceptionally dry winters.
The drought was declared after the Environment Department (Defra) brought together water companies, farmers and wildlife groups to discuss potential water shortages.
The Environment Agency says the East and South East will need well above average rainfall during the next few months for groundwater levels to recover.
Following the summit, Thames Water warned there was a high chance of water restrictions such as hosepipe bans this summer, unless there was significant rainfall or customers used less water.
Some rivers and groundwater levels are lower than during 1976 - the hottest year on record.
In the South East, Ardingly reservoir in West Sussex and Bewl in Kent are around two-fifths of their normal levels, according to the Environment Agency.
Southern Water has applied for a drought permit to help refill Bewl reservoir, while late last year South East Water was granted a drought order to help refill Ardingly.
Anglian Water has also been issued with two drought permits to refill two of its reservoirs.
Speaking after the summit, Environment Secretary Caroline Spelman said: "Ensuring we have enough water this summer is vitally important, and that is why I called the summit today.
"It is not just the responsibility of Government, water companies and businesses to act against drought. We are asking for the help of everyone by urging them to use less water and to start now."
Richard Aylard, sustainability director for Thames Water, said the Thames Valley and London had received below-average rainfall for 18 of the past 23 months.
"None of us know how much rain we're going to get this year, so to plan for the worst and hope for the best is a very sensible move."
Mr Aylard urged people to think about how much water they use.
He added: "There is a high chance we will need restrictions at some stage this
summer unless either we get a lot of rain or fantastic co-operation from
customers using less water."
Thames Water says people can save water with simple measures, such as turning
off the tap while cleaning their teeth or taking shorter showers and only washing full loads of laundry.
Flows in the River Lee, which goes through Hertfordshire and northeast London, are only 24% of the long-term average, while the Kennet in Wiltshire has seen flows of 31%.
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 Kennet has dried up completely to the west of Marlborough.
The Angling Trust 's chief executive Mark Lloyd said the Kennet should be "10 to 15 yards wide but it's a pile of tones that you can walk across in ordinary shoes".
Mr Lloyd said few people realised how grim the situation had become.
"The vast majority of people are unaware that we are in the middle of a crippling drought - river levels are lower in many areas than they were in 1976.
"Rivers and wetlands are without doubt the most important wildlife areas left in Britain and water levels are vital to support many species that rely on them."