The plant would be by the River Thames in east London

The plant in Beckton, east London, will start producing water sometime in 2009, in times of drought or low rainfall.

The site will provide up to 140m litres of drinking water a day - enough for nearly one million people.

But critics say the plant is a "sticking plaster" solution to the water crisis.

Planning permission for the plant was granted by the Department of Environment, Food and Rural Affairs and the Department of Communities and Local Government.

It is expected to run for up to 40% of the time over the next 25 years but critics say it is unnecessary.

Climate change

Richard Aylard, from Thames Water, said: "The desalination plant is a vital part of our plans to secure future water supplies to the capital.

"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."

But environmental campaigners condemned the government's approval of the plant.

"It is a sticking plaster solution to the water crisis we have in the south-east," said Rob Oates from the World Wildlife Fund.

"The government should instead conduct a bigger, strategic review of people's water usage and work to reduce demand and leakage, introduce metering in homes and encourage residents to install water-saving technology."

He added: "At the moment, half of the drinking water supplied to homes is used for flushing toilets and washing dirty clothes, which is madness."