The Mayor of London said that dramatic action was needed to prevent an acute water shortage and painted a vision of standpipes in the streets if nothing is done.

Mr Livingstone made the comments while inaugurating a public education campaign to promote water conservation. He said that he could ask ministers for an immediate hosepipe and sprinkler ban if the situation deteriorated. Modifying lavatory habits was an important part of this programme, he said.

“The quickest and most dramatic impact is, don’t use a sprinkler or hose in the garden, don’t use a hose to wash your car and don’t flush the lavatory if you have just had a pee,” he told a press conference at City Hall yesterday.

Londoners use an average of 165 litres of water every day, higher than the national average of 150 litres and about one third higher than other European cities.

Climate change, population growth and lifestyle changes are placing increasing demands on London’s water supply. Mr Livingstone also said that he would ask the Government for powers to bring in “compulsory water-metering” in the capital to reduce water consumption in the long term. Demand could outstrip supply by 6 to 10 per cent by the end of the decade, he said.

“I want Londoners to make small changes to save water without affecting their quality of life,” he said. “We need to take action now to better manage our water supply, so that we can avoid shortages and mandatory restrictions in the future.”

Flushing lavatories put a particular burden on the water system. The standard domestic facility uses 7.5 litres of water per flush. New dual-standard lavatories use only 4.5 litres when partially flushed. But a spot inspection by The Times of the lavatories at City Hall revealed two urinals in the gents’ flushing continually even when not being used for long stretches.

In the cubicles, only one out of four lavatories was unflushed in the men’s, and one out of seven in the ladies’, although a sign informed users in both that they used recycled water.

Roger Evans, the Conservative environment spokesman on the London Assembly, said: “If this 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?”

Visitors to City Hall yesterday were also appalled. Chandra Charma, from Kent, said: “That’s disgusting. If water’s such an issue, how come their grass is always kept nice and green?”

Discussions about when to flush have often been hampered by a very British embarrassment at the subject. During the last water shortage the headmistress of Haberdashers’ Aske’s School in Elstree, Hertfordshire, told pupils at assembly: “If it’s yellow, let it mellow; if it’s brown, flush it down”.

At last year’s British Invention Show in London, the top prize went to the man behind a device that is fitted to lavatories that makes them flush only while the handle is pressed. Judges praised the Interflush system as the biggest water-saving device in years. The device saves 47 per cent of the water usually used in the flush and can cut £50 a year from household water bills.

In the US, urinals have started to appear in home bathrooms, frequently at the instigation of the man, who promises that it will help to keep the floor dry.

Ozzy Osborne has one in his California home, as does Curtis Martin, of the New York Jets, who recommended them to men so that their wives would not nag them about lowering the seat.

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