The gigantic QEII reservoir in Walton is set to be drained later this month, bring road closures in the area during the summer, Thames Water have said.

Walton Road will be closed from April 26, the water company said, as they seek to drain most of the staggering 19,623 million litres of water (8,000 Olympic swimming pools) there in line with legal requirements.

Indeed, pretty much everything about the QEII Reservoir is big.

It provides an estimated 10 per cent of London's water supply, and is also home to one of the world's biggest floating solar farms.

Thames Water said that during the drainage their engineers would also be working to reline the two tunnels which feed water into the reservoir from the River Thames and carry it from there to the nearby Walton and Hampton treatment works, from where it is pumped to homes across London.

They hope the relining will strengthen the tunnels and prevent any future leaks.

Rachel Whiteman, Thames Water’s project manager, said: "The QEII plays a really important role in the supply of top-quality drinking water for millions of people across Surrey and London so we have to make sure it’s safe and in good working order.

"We’re carefully managing the rest of our reservoirs in London to keep taps flowing so customers won’t see any impact on their water supply."

Engineers will enter the tunnels through access shafts which will be constructed above each tunnel.

One of the shafts will be within the boundary of the reservoir site while a second will be in Walton Road, meaning the road will be fully closed for two months from April 26 and down to a single lane with temporary traffic lights from June until December, Thames Water said.

Whiteman added: "We’re sorry for the disruption the road closure will cause. We’ve worked with Surrey County Council to prepare a safe diversion route and urge road users to follow it rather than taking potentially dangerous shortcuts."

Work to drain down the reservoir started this week and is expected to take until June at least.

The water will meanwhile continue to go to the treatment works and on to customer taps to avoid any wastage.

Thames Water said around six meters of water will be left in the bottom of the reservoir to support wildlife including nesting birds, while the solar panels which float on the surface and generate renewable energy to power the treatment works can also stay in place.