As tire bans were imposed on a million more people in southern England, Thames Water admitted a major water desalination plant opened by the Duke of Edinburgh in 2010 will not be operational until next year. The company had repeatedly assured regulators as recently as January that the plant in Beckton, east London, was ready to go “for drought use”, claiming it would help provide fresh drinking water to up to 400,000 households every day . But The Telegraph has learned that the plant – the only one in the UK designed to turn salt seawater into fresh water – has actually been switched off amid questions about whether it was ever fully operational. Instead, Thames Water has asked ratepayers to stop washing their cars and watering their lawns ahead of an expected ban on hosepipes in the capital. Senior industry sources said the plant has been effectively mothballed amid concerns about its high operating costs compared to asking customers to cut back on water use. Thames Water made a profit of £488m last year. She has increased bills by more than 4% this year, with the cost of a family home in her area rising to around £415 a year. On Wednesday, a source at Ofwat, the water regulator, said it would review Thames Water’s management of the plant during this year’s shortages. It comes as bans were imposed on households across Sussex and Kent, with water companies blaming the driest July on record.