Britain's biggest water company has urged customers to stop using hosepipes as the country sweats through its third heatwave of the year with 35C highs expected.Thames Water issued a warning to the 16million people it supplies across Greater London, the Thames Valley, Surrey, Gloucestershire, north Wiltshire and west Kent.The beleaguered utility company confirmed it was 'asking customers not to use hosepipes or sprinklers during heatwaves' in an attempt to maintain water supplies.Thames Water is facing high demand after a very dry few months for south-east England and said three billion litres a day of water was used in the June heatwave.London last saw rain ten days ago on June 27 when thunderstorms hit, and there are no downpours in the Met Office's forecast for the capital over the next week.Thames Water is trying to avoid nationalisation as it struggles with nearly £20billion of debt and has faced criticism for its performance, sewage discharges and leaks.The firm has stopped short of imposing a hosepipe ban so far this year but is asking customers to reduce usage and delay 'non-essential outdoor jobs' involving water.Bosses said in a notice to customers: 'At times, we're using water faster than we can deliver it through our network. We're working around the clock to manage this, moving water to where it's needed and fixing leaks as quickly as possible. Bewl Water, the largest reservoir in south-east England, near Lamberhurst in Kent on July 1