Millions of people will be affected by the first hosepipe ban of the summer in England.
United Utilities (UU) said a temporary ban affecting seven million people in the north-west of England from 5 August would “safeguard essential supplies”.
It said reservoir levels were already low and that hot weather was forecast for the rest of July.
Other water companies across England said they had adequate supplies and had no plans to impose similar bans.
Carlisle and north Eden Valley, which have reasonable supplies, are exempt from the UU ban in north-west England.
The two areas receive water from local water sources – not the network which serves the rest of the region – and have not been as badly affected by the lack of rainfall, the firm said.
Earlier this month UU said it was moving water from Wales to boost depleted supplies across north-west England.
A hosepipe ban was introduced in Northern Ireland at the end of June.
Image copyright PA Image caption Low rainfall and hot weather has seen water levels fall at Dove Stone Reservoir on Saddleworth Moor
Martin Padley, from UU, said the ban, known as a temporary use ban, comes during what is believed to be the longest heatwave since 1976.
He said: “Despite some recent rainfall, reservoir levels are still lower than we would expect at this time of year and, with forecasters predicting a return to hot dry weather for the rest of July we will need to impose some temporary restrictions on customers.
“We are enormously grateful to customers for having helped reduce the demand on our network over the last couple of weeks but unless we get a period of sustained rainfall before 5 August these restrictions will help us safeguard essential water supplies for longer.”
The firm said the ban was alongside its other efforts to maintain essential supplies, including maximising water abstraction from ground water supplies, moving water around its regional integrated network of pipes and running a campaign to encourage customers to use water wisely.
The firm said it had the power to impose fines of up to £1,000 for people who flouted the ban.
Although water companies in other regions of England urged customers to continue to be “water wise” in the hot weather, they said they had no plans to impose similar bans.
Severn Trent Water, which serves more than 4.2 million homes and businesses in the Midlands and mid-Wales, said its reservoirs were in a “healthy position”.
South Staffs Water, which supplies more than a million customers in the Midlands, said: “The water levels in our reservoirs are healthy – we’ve got no issues.”
Temporary use bans – dos and don’ts
Hosepipes or sprinklers for watering private gardens and washing private cars are banned.
Watering private gardens with a watering can and washing vehicles using a bucket and sponge, which uses a fraction of the amount of water a hosepipe or sprinkler uses, is permitted.
According to UU, a hosepipe uses 540 litres an hour, as much as an average family-of-four would use in one day, while a sprinkler left running overnight uses as much water as a family-of-four would use in one week.
The ban can reduce water usage by 5-10%, according to research by United Kingdom Water Industry Research, which would amount to more than 100 million litres per day in the north west.
Anglian Water said its reservoir levels were where they should be or slightly above average for the time of year.
Northumbrian Water, Bristol Water and Thames Water all said they were not planning hosepipe bans.
Wessex Water, which said there was “no prospect” of introducing the region’s first hosepipe ban since 1976, said it was still important for customers to do their bit to save water.
Its head of water resources Aimee Shaw added: “Saving water can be as easy as taking a break from your chores, leave the grass to turn brown, don’t worry about washing the car or perhaps do one less load of washing in the machine.”
Welsh Water said it was encouraging its three million customers to use water wisely, even though it may soon start to rain.
The company warned that, after prolonged dry weather, it would be difficult for rain to penetrate the ground and help restore reservoir levels.
Bosses of England’s nine privatised water companies banked £58m in pay and benefits over the past five years, according to research by the GMB union published in June.
Privatised water firms in England typically lose between 20% and 22% of supply due to leakage, according to figures from Ofwat, the water industry regulator.
Professor David Hall from the University of Greenwich, an expert on the water industry, said privatised water firms had an incentive not to fix too many leaks as the cost of work would bite into profits and become uneconomic.
“They can’t recoup the cost of making reductions in leakage levels except by reducing profits, that’s not what they want to do,” he said.
“If the leakage levels were not so high, the daily volume of water delivered would be higher, therefore we would reach the point of hosepipe bans much later.
“Any restriction on water use is a restriction on people’s quality of life.”