Londoners are being urged to flush their toilets fewer times a day to help save water.
The capital uses more water than the national average
The city uses more water than the national average and a third more than other European cities.
London mayor Ken Livingstone said they could start by "not flushing the toilet after they had a pee" and taking a shower instead of a bath.
He also wants to make water meters compulsory to reduce consumption, which he claims will outstrip supply by 2010.
Londoners use on average of 165 litres of water every day, higher than the national average of 150 litres.
Most of this water is used for washing and flushing the toilet.
Mr Livingstone's comments, during his weekly press conference, come after the Thames region received just 24% of its average rainfall for June.
He said: "Water is a precious resource that we cannot afford to take for granted.
Londoners use 165 litres of water each every day whereas Berliners use just 120 litres
A full bath uses 90 litres of water but a quick five minute shower uses only 30 litres
A tap that drips once every second wastes four litres of water a day.
"Climate change means there will simply not be enough water resources to support inefficient use in years to come.
"We need to take action now to better manage our water supply. Garden sprinklers use as much in an hour as a family of four uses in a day."
But London Assembly Conservative Environment spokesman Roger Evans said the Mayor should have tackled the issue long ago.
Ken claims supply will outstrip demand by the end of the decade
"Water use is clearly an important issue but the Mayor is being a tad alarmist with his request that Londoners do not flush the toilet unless it is absolutely necessary," he said.
"If the issue of water usage is so great that we have to consider such unappealing solutions then why has the Mayor waited so long to act."
Mike Tuffrey, for the Liberal Democrats, said the Mayor ignored the fact that almost 1,000 Olympic size swimming pools of water were lost through leaky pipes.
"While steps to reduce demand for water, such as compulsory water meters, are welcome the approach to solving this problem must be led by the water companies," he said.
A poster campaign featuring water saving tips will run across the London Underground.