Southern Water, South East Water, Thames Water, Anglian Water, Sutton and East Surrey, Veolia Central and Veolia South East are understood to be introducing restrictions on 5 April.
The Environment Agency says it anticipates a severe drought this spring and summer as the window for recharging groundwater and reservoirs has passed.
In a drought prospect report published today, the agency says parts of south east Yorkshire and the West Country are also at risk of drought.
Customers in the regions covered by restrictions are prohibited from watering gardens, filling paddling pools or washing cars with a hosepipe.