Britain's biggest water company will ban hosepipes and sprinklers from next month, the firm has announced.
The South East has experienced its driest period for more than 80 years
Thames Water, whose eight million customers will be affected by the ban, says two unusually dry winters have caused "serious" water shortages.
The ban is the first in 15 years for the firm, which operates across the Thames Valley, in London, and from Kent to Gloucestershire.
Hosepipe bans are already in place throughout much of south-east England.
Thames Water's announcement follows decisions by Sutton and East Surrey Water, South East Water, Southern Water, Mid Kent Water, and Cholderton and District Water to ban hosepipes.
And earlier this month, ministers granted Folkestone and Dover Water the power to install compulsory metering in homes it serves. Other firms are considering similar applications.
Thames Water chief executive Jeremy Pelczer said the decision had not been taken lightly.
"The drought across the South East has now gone on for so long that we have to be prudent and introduce measures that will make best use of limited supplies and help protect the environment," he said.
Mr Pelczer said the move would "lessen the likelihood of more stringent restrictions later", but much depended on rainfall levels.
The ban comes despite the latest figures for water storage showing Thames Water reservoirs to be 96% full - the second fullest in the country.
"Our reservoirs are okay," a spokesman told the BBC News website.
"Where we've got the problem is with the low water levels in the aquifers."
Aquifers - also known as ground reservoirs - are underground layers of rock that collect water.
The spokesman said aquifers provided "a significant" amount of Thames Water's supply.
The South East has had two consecutive winters with below-average rainfall.
The period between November 2004 and January 2006 was the driest for more than 80 years, surpassing even the notable drought of 1974-76.
A spokesman for the Environment Agency supported the Thames Water's move, saying it had "acted responsibly by introducing a hosepipe ban at this time".
And Andrew Marsh, of the Consumer Council for Water, said imposing hosepipe bans was a "sensible precaution".
"Consumers will support the ban, but the problem with Thames Water is that they have a pretty poor record with leakage."
Darren Johnson, chair of the London Assembly's Environment Committee, was also critical of the company's record.
"Thames Water, Ofwat and the Government have got to agree a long-term investment strategy to repair and replace London's crumbling mains system as a matter of urgency," he said.
Thames Water is currently spending £1bn replacing Victorian pipes, which are estimated to leak around a third of water travelling from reservoirs to homes.