South East Water’s Ardingly Reservoir serves 228,000 customers in Haywards Heath and the surrounding areas in Sussex.
A hosepipe and sprinkler ban, which was imposed on the area in July, will remain in place until water supply is secured, the EA said.
The water company said from Friday watering new lawns at domestic premises, trees and hedges planted within the last three years and food crops at domestic premises and private allotments are no longer exempt from the hosepipe ban.
Eastbourne has been included in the drought area despite its water being supplied by Arlington Reservoir.
The area of Sussex adds to others across the country that entered drought status earlier this year – Yorkshire, Cumbria, Lancashire, Greater Manchester, Merseyside, Cheshire, East Midlands, and the West Midlands.
The National Drought Group – which includes the Met Office, government, regulators, water companies, the National Farmers’ Union, Canal & River Trust, anglers, and conservation experts – continues to monitor the situation and will next meet on 30 October.