The Environment Agency made the announcement on Friday, October 1, saying it has changed the status of the areas around Haywards Heath and Eastbourne to drought.
It said this is due to declining water levels at Ardingly Reservoir.
It confirmed that Hampshire, the Isle of Wight and the rest of Sussex and Surrey remain in 'prolonged dry weather status'.
South East Water is the main water company supplying East Sussex (including Eastbourne), but Southern Water serves the Brighton, Lewes and Hastings areas. SEW also serves areas in Mid and West Sussex, including Haywards Heath, Burgess Hill and Heathfield. The Horsham District is supplied by Southern Water. People can check who their supplier is at www.water.org.uk/customers/find-your-supplier.
Richard O'Callaghan, area environment manager at the Environment Agency said: "Autumn and winter may feel like there is an excess of water at hand, but the climate is changing, and we must be sensible about water use all the time, not just some of the time. A few bursts of recent rainfall do not undo several dry months of cumulative impact.
"It will take sustained rainfall to address prolonged dry weather and replenish reservoirs and rivers. Everyone can help speed that up by being sensible with their water use."
The Environment Agency said each drought is different and can be classed as environmental, agricultural or public water supply drought. In Sussex, the hot and dry weather earlier this year has impacted availability of supplies, meaning further action is required to conserve them.
Moving to drought status means the Environment Agency and water companies will continue to implement the steps of their pre-agreed drought plans to manage the impacts.
The Environment Agency's announcement said: "The change in status reflects the decline in water levels at Ardingly Reservoir, near Haywards Heath, and South East Water's drought order application today to help conserve the reservoir's remaining water.
"Ardingly Reservoir's water level has rapidly dropped since June and is currently at 27.6 per cent total capacity. The low reservoir level has been driven by the combination of a dry spring and early summer and higher temperatures associated with climate warming, which has driven very high demands.
"Defra and the Secretary of State will determine the drought order application and the Environment Agency, as a regulator, will provide technical advice assessing the needs of the people and the environment.
"The Environment Agency is working closely with South East Water to ensure it is delivering appropriate actions within its drought plan. Exemptions have been removed from the company's hosepipe ban which will remain in place until water supply is secured. All customers are urged to continue following water-saving advice."
Other areas of the UK that entered drought status earlier this year include Yorkshire, Cumbria and Lancashire, Greater Manchester Merseyside and Cheshire, East Midlands and the West Midlands. The National Drought Group - which includes the Met Office, government, regulators, water companies, the National Farmers' Union, the Canal & River Trust, anglers, and conservation experts - is monitoring the situation and will meet on Thursday, October 30.
South East Water confirmed that it has applied to the Secretary of State for Environment, Food and Rural Affairs for a Drought Order.
SEW's head of Water Resources Nick Price said: "This year has been exceptionally dry, and the River Ouse catchment in Sussex has only experienced two years (1976 and 1893) with lower spring and summer rainfall in its 155 year rainfall record. Consequently the River Ouse, where we take water from in Sussex for supply and to refill our Ardingly Reservoir, is running low.
"Therefore we are applying to the Secretary of State for Environment, Food and Rural Affairs for a Drought Order to help conserve the remaining water that is stored within our Ardingly Reservoir.
"We are working closely with our regulators and following our Drought Plan which sets out the steps we take during periods of low rainfall. We would like to thank our customers who have also helped, by being mindful of their water use where possible, and appreciate their support in keeping demand down.
"To conserve as much water as possible we are also planning to remove some of the exemptions currently in place as part of our Temporary Use Ban, or hosepipe ban, for customers in Sussex. This action will come into effect on Friday 17 October."
South East Water said its Ardingly Reservoir serves 228,000 customers in Haywards Heath and the surrounding areas in Sussex.