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Plans to build the UK's first desalination plant have been blocked by London's mayor.
Ken Livingstone said the proposed £200m (US $383 million) desalination plant by the River Thames in Beckton, east London, would consume too much energy.
Thames Water, which is considering an appeal, says the plant is essential in ensuring London has enough water for its growing population.
Rejecting the proposal the mayor said: "I am not convinced that a desalination plant is a viable long-term way to ensure London has an adequate supply of water.
"With increasing concerns over the damage we do to our environment and climate change, it's important that we reduce rather than increase the levels of energy we use in London Like any city London does not have unlimited natural resources and we need to educate people to be more careful to avoid wasting our water both at home and at work," Livingstone said.
A spokesman for Thames Water said: "We are concerned that, without new sources of water, we will be unable to meet increasing demands.
"The plant has been designed to play a key role in guaranteeing water supplies to customers during drought periods."
Thames Water had planned to use reverse osmosis to remove salt from the Thames.
The company is in the process of replacing 850 miles of water mains over the next five years. Company officials also are considering plans to build a new reservoir near Oxford to store surplus winter river flows, to ensure London residents have enough water in the future.