Britain's Prince Philip opened the country's first desalination plant Wednesday, which aims to provide much-needed back-up for water-stressed London and its current leaky network.

The plant in Beckton, east London, will be able to turn a mixture of seawater and river water from the River Thames into high-quality drinking water for up to one million Londoners.

"When the next drought arrives, it will ensure that we will always be prepared," project manager Steve Baldwin explained during a tour of the facility.

"It is more expensive to use," Baldwin said about the plant that cost over 270 million pounds (more than 320 million euros/390 million dollars). "We accept that, but we have got no choice."

The 2005-2006 drought was "too close for comfort", the chief executive of Thames Water said in a statement. "Existing resources -- from non-tidal rivers and groundwater -- simply aren't enough to match predicted demand in London."

London was not as "rainy" as people may believe, Martin Baggs said, with the city getting about half as much rain as Sydney, and less than Dallas or Istanbul.

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

Former London mayor Ken Livingstone has worried that the plant may risk encouraging Londoners to waste water.

Instead, the ex-mayor has said Thames Water should speed up the modernisation of London's leaky water distribution network.

Darren Johnson, Green Party member of the London Assembly warned that "a desalination plant is very energy intensive," and urged Thames Water to "put more efforts in tackling leakages."

"We need to be reducing the amount of water that we waste, both as individual households and Thames Water as a company, rather than simply looking at increasing the supply," he told national radio.