Billions of litres of extra water is being readied to pump into the south of England as Britain faces another heatwave with temperatures to straddle 30C (86F) this week.
Some 450 million litres of water per day is on standby - enough to fill 900,000 paddling pools, according to Thames Water.
On a hot day, water consumption in London increases around 14% and this shoots up to 27% in Thames Valley as people escape the capital, the company said.
Alex Burkill, a meteorologist with the Met Office, said Britain would start to heat up on Monday and peak on Tuesday and Wednesday with temperatures in the high 20s and breaking 30C in some places.
It will be hottest in the south and south-east of England, but will fall short of mid July's record this year of 33.5C (92F), he said.
Mark Jenner, head of operations systems at Thames Water, said: "We have a team of specialists who weather-watch throughout the year and use their years of experience to estimate how much water our customers will use during a hot spell.
"In some places demand for water in the evening nearly triples, so it's up to us to do the maths, put more water through our treatment works and get that extra water into supply."