Emergency bottled water deliveries are being set up as around 1,500 customers were affected by a water supply problem on Sunday (July 15).
Southern Water said that a "technical issue" at Vinings Water Treatment Works in West Sussex meant some of its customers were "likely to shortly have interrupted water supplies."
The water company said it had implemented emergency plans in light of the issue.
It tweeted: "While we work hard to fix the issues & restore normal supplies we’re currently setting up manned bottled water stations & we’ll let you know where they’ll be shortly.
"We’re also tankering in extra water to limit the number of customers affected & are organising bottled water deliveries for our customers in vulnerable circumstances in the affected areas.
"We’re really sorry because we know how inconvenient it is to be without water, particularly in this extremely hot weather & thanks for your patience & understanding while we work to restore water supplies to normal."
A Haslemere postcode has once again been affected by the shortage – earlier this month parts of the town had either no water or very low pressure after a power failure at the Blackdown Reservoir .
List of affected postcodes