Londoners are being urged to stop using hosepipes as another heatwave grips the capital.
Thames Water said it was struggling to keep up with demand at times as the capital frazzles in temperatures spiralling towards 34C.
The last rain in London was nine days ago on June 27, when thunderstorms erupted over the city.
More thunderstorms could hit the city on Friday or Saturday, says the Met Office.
But if they do not the next precipitation, and then only drizzle, could be on Saturday, July 18.
So the parched capital could go 20 days or longer without a proper drop of rain.
Thames Water, which has been heavily criticised for its performance on sewage and leakages, stopped short of imposing a hosepipe ban.
But it appealed to its 16 million customers in London, the Thames Valley and other areas not to use their hosepipes to water their gardens, clean their cars or for other purposes.
Andrew Tucker, water demand reduction manager at Thames Water, said: “All our water is drinking-quality and comes from local sources, so small changes, such as using a watering can, reusing water and delaying non-essential outdoor jobs, can reduce what we need to take from the environment.
“We are asking customers to avoid using hosepipes and sprinklers on lawns and gardens so that supplies can be maintained for everyone during periods of exceptionally high demand.”
The company stressed that at times water was being used “faster than we can deliver it through our network”.
The water chiefs also urged customers to avoid filling or topping up paddling pools unless necessary, and to delay washing cars and other vehicles.
Customers were also advised to turn off taps when they are not in use, and to only use the “water you need and avoid unnecessary waste where you can”.
The company stressed that during the record-breaking heatwave in June, water use soared to three billion litres a day.
This spring was drier than usual, it added, with much lower rainfall in the region, with April seeing only 20% of what would have been expected.
The Met Office is forecasting than temperatures in London could reach 34C on Monday and Thursday this week, and staying above 30C until at least Sunday.
In Kent, South East Water has imposed a hosepipe ban
It said: “Due to sustained high temperatures and near record levels of demand for treated drinking water across Kent, we have made the difficult but necessary decision to introduce a Temporary Use Ban (hosepipe restrictions) for customers in our Kent region.
“While we’ve worked to sustain supplies, the continued pressure on our network means we now need to reduce non-essential water use.”
Yellow heat health alerts have been issued by the UK Health Security Agency for London, the South East, the South West and across the Midlands until 8pm on July 11.
The agency said that within the warning area, minor impacts were likely across health and social care services because of the high temperatures, including increased use of healthcare services by vulnerable people and a greater risk to life to the same group.
