Plans to build the UK's first plant which would turn salt water into drinking water are to be scrutinised at a public inquiry.
The plant would "guarantee" water during drought periods
Mayor Ken Livingstone overturned Newham Council's approval for the desalination unit in Beckton, saying the energy used would add to global warming.
Now Thames Water is to appeal saying the plant is essential to cater for London's needs.
The inquiry at City Aviation House in east London is due to last five weeks.
If the £200m plant gets the go-ahead it would not be ready until 2009 at the earliest, which Mr Livingstone said was too late.
The plant, which would be used mainly during droughts, would be the first in London to take water from the tidal stretch of the Thames, using the desalination process to remove salt from water.
The treated water would then be pumped through a new pipeline to a reservoir in Woodford, east London, ready for distribution to customers across north-east London.
It could supply 140 million litres of water a day, enough to supply 400,000 homes.
But Mr Livingstone believes Thames Water should be spending its resources on fixing leaks in the existing water network and reducing demand for water by installing water-saving devices and educating customers.
Speaking on the BBC Radio 4 Today programme, Mr Livingstone's environmental advisor Alex Bax said: "Most other forms of the way we get water in the UK use rather less energy than this will.
"If Thames just achieved anywhere near industry best practice in terms of fixing leaks, we wouldn't need the plant."
But Thames Water environment director Richard Aylard told the programme: "We are driving down leakage as fast as we possibly can."
He said London's pipes were very old and 250 miles of pipes were replaced every year.
"We have looked hard at all the options and the only way to bridge this gap quickly is with this desalination plant," he said.
Roger Evans, environment spokesman for London Assembly Conservatives, said: "We desperately need to increase the availability of clean water for Londoners and it's shocking to see Ken Livingstone opposing this on spurious environmental grounds."
The Green Party on the London Assembly said it backed Mr Livingstone's decision to overturn planning permission.
It said the plant would pump out 25,000 tonnes of greenhouse gases each year but produce a relatively small amount of water.
HOW REVERSE OSMOSIS DESALINATION WORKS
1. Water flows in from the estuary or sea
2. Salt water contains sodium and chloride ions
3. Pressure is applied to force salt water through membrane
4. Semi-permeable membrane with millions of microscopic holes
5. Clean water fit for drinking
6. Saline concentrate flows out