SEW's chief executive David Hinton said the heatwave meant drinking water was being taken from its storage tanks faster than it could be treated and refilled.
"To make sure we can maintain a consistent public water service and protect essential supplies for hygiene, drinking and cooking for everyone, especially our vulnerable customers, we must reduce overall demand," it added.
Hinton, who resigned in May following supply issues but remained in the post to allow for an "orderly transition", said he recognised customers' frustrations due to the ban.
"We are deeply sorry for the disruption this causes, and we are incredibly grateful to everyone helping us protect Kent's water."
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