A hosepipe ban will be announced for London within days and come into force next week, the Standard can reveal.
Thames Water will formally confirm the measures later this week, it is understood.
Mayor Sadiq Khan said the hosepipe ban was “inevitable” and warned it could be in place for weeks unless Londoners were “good citizens” and limited their use of water.
It came as London Fire Brigade on Monday admitted it was being “stretched to the limit” by 200 wildfires at the weekend - 10 times more than normal for the time of year.
Officers said there had been 340 grass fires in the first week of August, eight times more than the same week last year.
This has required firefighters to spend hours in arduous conditions bringing the blazes under control. Many crews have been dispacted well outsuide of their normal “ground” - with a crew from Maldon in Essex among those attending a wildfire in Chingford.
There are concerns that thunderstorms forecast in the capital for Wednesday and possibly Tuesday could result in flash flooding, while high winds may cause wildfires to spread even more rapidly in bone-dry conditions caused by record temperatures over the last month.
Thames Water, the country’s largest water firm, will ban its 15 million customers in London and the Thames Valley from watering gardens or washing cars and ask them to conserve supplies.
The restrictions follow a request last month, as the heatwave began, for Londoners to take shorter showers, not to run the tap while brushing teeth and limit the use of garden hoses.
However Thames does not have a record of taking a hard-line approach to enforcing bans, and breaches are unlikely to result in fines.
A Thames Water spokesman said: “The prolonged hot weather and ongoing lack of rain has meant that we are now planning to take our drought plan to the next stage which is to introduce a temporary use ban.”
Thames has been criticised for leaking almost a quarter of the 2.6 billion litres of water it supplies daily but says it fixes 1,200 leaks a week and aims to reduce the leakage rate to a fifth by 2024/25.
Mr Khan told the Standard: “Many Londoners will be angry that they have got to make a personal sacrifice when it appears that the water companies, who make huge profits, and pay their shareholders massive dividends, aren’t addressing at source issues around leakages of water.
“The water companies have got to sort their house out in relation to preventing water leakages and improving infrastructure and, when there is a leak, making sure it is fixed sooner rather than later.
“But we also have to reduce our use of water. It is a precious, finite resource. The drought combined with the hosepipe ban could be here for some time if we don’t make personal sacrifices. We have got to be good citizens to help us all through this difficult time.”
South West Water on Monday announced a hosepipe ban for Devon and Cornwall — its first in 26 years. Welsh Water, Southern Water, and South East Water have already imposed bans, while Yorkshire Water’s will start next Tuesday.
A yellow warning of thunderstorms has been issued for London tomorrow and Wednesday, with “torrential rain” potentially bringing disruption to roads and the Tube network and some homes are at risk of flooding.
Mr Khan was on Monday visiting the scene of two recent wildfires in Epping Forest as he appealed for Londoners not to have barbecues or risk sparking blazes in tinder-dry parkland. A third broke out just before his arrival - which required firefighters to hose down trees and shrubs as the mayor gave media interviews.
Two hectares were destroyed near Hollow Ponds in Epping Forest last Friday — only days after a nearby blaze in the forest.
On Sunday, firefighters spent more than four hours tackling a grass fire in fields surrounding a school on Turkey Street in Enfield. Two horses were rescued but five hectares of heathland was destroyed.
On Saturday, about 70 firefighters spent almost six hours tacking a grass fire on Rammey Marsh in Enfield, which caused smoke to spread across the M25.
Mr Khan thanked crews for their efforts and said it was important that Londoners were “not lulled into a false sense of security” if rain did arrive, as the risk of wildfires remained until the grass turned green.
LFB assistant commissioner Jonathan Smith said: “Over the last few weeks it has been relentless, firefighters have been working in very challenging conditions and extreme heat... our resources are being stretched.
“We need the public to act too. Don’t barbecue in open spaces or on balconies, don’t discard your litter and glass and make sure you put your cigarettes out properly. We aren’t trying to be killjoys, we are trying to protect you, your homes, your parks and open spaces and wildlife.”
Meanwhile, the body of a man who entered the Thames has been recovered by police divers. An emergency response had been launched at around 4pm yesterday after the man — believed to be in his early twenties — was seen entering the water at Hampton Court. His body was recovered at about 10.30pm.
Superintendent Richard Smith, the Met’s acting commander for the South-West, said: “The danger and risk of loss of life is incredibly real. Please do not enter the water at any point. The consequences are enormous and we do not want to see any more families having to suffer such terrible news.”