The Duke of Edinburgh was on hand to unveil the plaque at the official opening of the UK's first desalination plant this week.

The Thames Gateway Water Treatment Works has been built in East London's Beckton and has been put in place to help alleviate pressures on the capital's limited water supplies.

The plant has been a long time coming and has not been without controversy.

It was opposed by London's previous Mayor, Ken Livingstone, on the grounds that it would be energy-hungry and the carbon associated with running it would add to the problems of drought facing the city.

It was finally given the go ahead after operators agreed to run the plant using only energy from renewable sources - mainly sustainably-produced biodiesel.

Now operators Thames Water say the plant will provide a much-needed back-up supply for a 'seriously water-stressed' London for use in event of drought.

The plant is capable of will turning a mixture of seawater and river water from the tidal Thames into high-quality drinking water for up to one million Londoners.

The key treatment process in desalination is reverse osmosis, which involves forcing salty water through extremely fine membranes.

This tried-and-tested technology is used at 14,000 water treatment plants across the world and has kept crews on Royal Navy ships refreshed for decades.

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.

Martin Baggs, Thames Water's chief executive, said: "People may wonder why we're equipping 'rainy' London with a desalination plant, something more often associated with the Middle East, southern Europe or ocean-going liners.

But the fact is, London isn't as rainy as you might think - it gets about half as much rain as Sydney, and less than Dallas or Istanbul. Water is an increasingly precious resource that we can no longer take for granted.

"Our existing resources - from non-tidal rivers and groundwater - simply aren't enough to match predicted demand in London.

"That's why we're tapping into the new and limitless resource of the tidal Thames, fed by the rolling oceans beyond, so we can ensure our 8.5 million customers have enough water in future in the event of a drought.

"The 2005/06 drought was too close for comfort, with only a very wet May saving the day, and we never want a repeat of that.

"It highlighted what we already knew: additional water sources are needed, as well as a lot more work on reducing leakage, to be sure we have sufficient supplies long-term.

"This new works is a major advance in desalination technology and in UK water resource management.

"Running it on biodiesel, derived from materials including used cooking oil, will also help us tread as lightly as possible on the environment, on which our core business depends."

Sam Bond