Water companies across southern and eastern England are to introduce hosepipe bans amid drought conditions.
Seven firms say they will impose water restrictions after two unusually dry winters left reservoirs, aquifers and rivers below normal levels.
Southern Water, South East Water, Thames Water, Anglian Water, Sutton and East Surrey, Veolia Central and Veolia South East are to enforce restrictions.
All seven companies said they will impose bans from 5 April.
The drought-affected areas are the south-east of England and East Anglia.
But the Environment Agency (EA) warns in a new report that the drought could spread as far north as East Yorkshire and as far west as the Hampshire-Wiltshire border, if the dry weather continues this spring.
The EA warns that drought conditions are expected to spread across more of England in coming weeks, unless strong rains arrive.
It will also warn of effects on agriculture that could raise prices of potatoes and other vegetables.
It says plans are in place to ensure that the Olympic Games will not be adversely affected, by using water from "sustainable supplies".
"The Olympic Park and other Olympic venues have a high level of resilience to meet their needs even during a drought," says the Agency.
It also added: "The Queen's Diamond Jubilee pageant at the beginning of June will not be affected by the drought."
A ban on hosepipes means they cannot be used on gardens, plants, cars or boats for "recreational use"; to fill or maintain ponds, pools or fountains; and to clean paths, walls, windows or other artificial outdoor surfaces.
People in breach of these terms risk being prosecuted and fined up to £1,000. Watering cans and buckets are still allowed.
Environment Secretary Caroline Spelman said the temporary restrictions would "help protect the public's water supply in the areas most affected by the record low levels of rainfall we have experienced over the last 17 months".
She said: "We can all help reduce the effects of drought by respecting these restrictions and being smarter about how we use water.
"Taking action now to reduce how much water we use will help us all in the future."
News of the hosepipe bans has coincided with warm weather in much of the UK, although the drought conditions are a consequence of successive dry winters.
BBC Weather meteorologist Nick Miller said: "This prolonged spell of mild or very mild weather that we've seen since mid-February with temperatures rising as high as 19C isn't helping the issue, nor of course is the current dry spell.
"The required sustained period of rainfall for the worse affected areas simply isn't in the immediate forecast."
Meanwhile, the area formally in drought is expected to extend beyond south-eastern counties, with parts of Yorkshire likely to be named as officially in drought.
Counties that have received much less rainfall in recent months also include Shropshire and Somerset.
The National Farmers Union has warned of the impact on both arable and livestock farming, and is asking for restrictions on agricultural water use to be avoided wherever possible.
Farmers have reportedly planted 80% of the area they usually fill with crops, and are expecting lower than average yields.
But conservation groups point out that if farmers, householders or businesses take more water from rivers and lakes that are already poorly supplied, wildlife will feel the effects.
The Environment Agency is issuing a wide range of advice. It says:
- Farmers should look for ways to share water resources by setting up water abstractor groups and to take steps now to improve water efficiency
- Farmers should top up their storage reservoirs, to ensure there are better supplies for the summer months
- It has introduced a fast-track process for farmers to apply to take additional water when river flows are high.
Reservoirs such as Bewl Bridge in Kent are below half of their normal level for the time of year.
Heavy rains could yet stave off the worst of the impacts, but forecasters are predicting drier than average conditions for the next few months.
In the worst case, this could lead to emergency measures such as supplies being limited to public standpipes in the street being implemented, as was the case during the 1976 drought.