HRH The Duke of Edinburgh opening the Thames Gateway WTW
The capital is classed by the Environment Agency as "seriously water-stressed", which means that demand could outpace supply in a long dry period.
With climate change threatening hotter, drier summers and an additional 700,000 people forecast to move to London by 2021, the new water works will be available to help provide the capital's supplies for the future - whatever the weather.
The key treatment process in desalination is reverse osmosis, which involves forcing salty water through extremely fine membranes. However, while most reverse osmosis plants have one or two stages, which yield around half of the source water as drinking water, the £270m Gateway works is the world's first-ever four-stage reverse osmosis system, yielding a far more efficient 85 per cent.
The works will only take in water on the outgoing tide, when it is a third as salty as normal seawater and so requires less energy to treat it.
The plant is able to produce 150 million litres a day of water when needed, enough to supply 400,000 homes. The water will be blended with other supplies, so up to 580,000 properties in northeast London (1.4 million people) will potentially receive it in varying proportions.
Watch our video on the Thames Gateway Water Treatment Works