A hosepipe ban introduced by Thames Water in August, banning its 15m customers from watering gardens or washing cars as it sought to conserve supplies after this summer’s record 40C temperatures, remains in place.

Ms Bentley said the desalination plant was only planned as a “swing site” for intermittent use, able to supply 100 mega-litres a day – five per cent of the capital’s needs.

She told the Standard: “It was operational in time for the Olympics. It’s been operated three times – it was never built as a permanent site. “Because of all the dry weather we have been having, we have been investing to get it operational again. “It’s a fascinating project but it needs a lot of maintenance. With all the dry weather and the risk of drought, we want that working for next summer.” The super sewer, known as the Thames Tideway tunnel, stretches from Acton to Abbey Mills pumping station in east London.

It will act as the “world’s biggest storm tank” when run-off water from thunderstorms threatens to overwhelm the capital’s existing Victorian sewers, built 150 years ago by Joseph Bazalgette. The final “drive” to Abbey Mills has been completed and engineers are now lining the 7.2m-wide tunnel and adding a secondary concrete lining.

It is due to be brought into use by the end of 2024 and should reduce sewage spills into the River Thames by more than 95 per cent. The Lee tunnel, part of the project, has already been used 37 times – preventing sewage from entering the river. Thames Water customers have begun paying a premium on their bills of about £20 to £25 a year over the next 80 to 100 years to fund the super sewer’s construction.