Britain gave the green light on Wednesday for a desalination plant on London's River Thames in an effort to cope with climate change and get clean water to the capital's booming population.

The plant, one of the biggest outside the water-starved Middle East, will produce 140 million litres of drinking water a day when it is up and running in 2010 as London's population heads over the nine million mark.

Thames Water, which will run the 200-million-pound ($410 mln) plant near the Thames Barrier protecting London from tidal surges, welcomed the decision and said it was one of a number of key measures to guarantee the clean water future of the city.

"With pressures such as climate change and population growth the plant is essential alongside our continuing progress in reducing leakage and proposals for a new reservoir in Oxfordshire," the company said.

Final political approval for the plant came after a three-year battle with London mayor Ken Livingstone who had argued that stopping the water leaking from London's 100-year-old network of pipes would be more effective.

In a terse statement, Livingstone said he was studying details of the approval, which only came after Thames Water set out in detail the circumstances under which the highly energy-intensive plant would be used.

Thames Water said it had cut water leaks to 702 million litres a day now from 915 million litres a day in 2004, the plant would be used intermittently and only use biodiesel and therefore be climate friendly.

But environment campaign group WWF rejected that argument.

"It is nonsense to imagine that London, or indeed anywhere in the UK, needs a desalination plant to supply its freshwater needs. What we really need to do is reduce leakage, which still stands at 25 per cent," said WWF's Rob Oates.

"This is the UK, not Yemen, and it rains here a lot. If some very simple technology were introduced to harvest and store rainwater we could cut demand for water... by 50 per cent."

More than half of Britain's domestic water usage goes straight down the drain via baths, running taps and toilets.

Scientists say southeast England -- which suffered a major drought last year with reservoirs running dry -- could become water-stressed due to a combination of changed weather patterns due to climate change and a rising population.

Watchdog Waterwise said Londoners use on average 165 litres of water a day each against the national average of 150 litres.

This compares with averages of 107 litres/day in Belgium, 131 litres/day in Denmark, 200 litres/day in Norway and 360 litres/day in the United States.

"With only slight changes in behaviour like reducing toilet flushes, taking less time in the shower and not leaving the tap running when brushing your teeth Londoners could easily cut consumption by 30-40 litres a day," a spokesman said.