Hosepipe bans affecting about 20 million customers have been introduced by seven water authorities in parts of southern and eastern England.
People who flout the bans, which follow one of the driest two-year periods on record, face fines of up to £1,000.
Suppliers Thames, Southern, South East Anglian, Sutton and East Surrey, Veolia Central and Veolia South East have all introduced "temporary use bans".
The government has urged householders to be "smarter about how we use water".
Using a hosepipe to water a garden, water plants, fill a pond or clean outdoor surfaces are all banned as are filling and maintaining ornamental fountains.
But exemptions are in place for grass and surfaces used for national and international sports which means the Olympic and Paralympic games will be unaffected.
Disabled people with blue badges are exempt, while some businesses, including car washing firms, will also be allowed to continue using hosepipes in most areas.
And some drip irrigation systems featuring perforated hoses are allowed.
Water companies say they have no option but to put the bans in place to preserve essential water supplies but say they also need their customers to help cut down on their usage.
Most of the suppliers expect the ban to last all summer.
Anglian Water managing director Peter Simpson said: "Two dry winters have prevented rivers, reservoirs and aquifers from refilling with the water we treat and supply the rest of the year, especially during the hotter months when demand rises."
Sutton and East Surrey operations manager Mike Hegarty, meanwhile, warned there was no end in sight to the situation.
"We have said from the outset that we very much regret having to impose this bar but this drought is becoming increasingly serious."
He added: "We have no choice if we are to protect our customers by ensuring the long-term security of their water supply."
Householders are being urged by the companies and the government to cut their water use with measures including taking shorter showers and washing fruit and vegetables in a bowl rather than under the tap.
In some areas, drought has left groundwater below levels in the 1976 crisis when household supplies were cut off and standpipes used.
The introduction of the bans follows the third-warmest March - and fifth driest - since records began in 1910.
Lincolnshire, Cambridgeshire, parts of Bedfordshire and Northamptonshire, and west Norfolk have been in drought since last summer.
Much of south-east England, including London, is also affected and parts of North, South and East Yorkshire have become the latest to be declared as officially in drought by the Environment Agency.
Tips for saving water
- Use dishwashers and washing machines only when they are full
- Use bathwater and washing-up water to wash the car and the garden
- Save the cold water that comes through before a tap runs hot, and use it to water plants
- Keep a jug of water in the fridge instead of waiting for the tap to run cold
- Turn off the taps when you're brushing your teeth or shaving
- Install a water-saving device in the toilet
- Grow your grass a little longer. It will stay greener than a close-mown lawn and need less watering
