Billions of litres of extra water is being readied to pump into regions facing yet another Summer 2016 heatwave, with temperatures to straddle 30C (86F) this week.
Some 450 million litres of water per day is on standby in south-east England - 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.
Britain will start to heat up on Monday and peak on Tuesday and Wednesday, said Alex Burkill, a meteorologist with the Met Office.
Temperatures will average in the high 20s and break 30C in some places, he added.
It will be hottest in both 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.”
On an average day Thames Water puts 2,100 megalitres (one megalitre is one million litres) into London’s water supply, and 550 megalitres into the Thames Valley.
On hot days, it puts in around 2,400 megalitres into the capital and 700 megalitres to the rest of its region.
The hottest day on record was 38.5C (101F) in Faversham Kent, August 10 2003.