Emergency drought measures allowing a company to take water from a river to boost reservoir supplies have been allowed by the Environment Agency.
Bewl Water reservoir reached its lowest ever level in late 2005
Southern Water confirmed on Thursday that its request for a drought permit at the River Medway had been granted.
The water firm said Bewl Water reservoir, on the Kent/Sussex border, supplied customers in Medway, Thanet and Hastings and the move was "vital".
Campaigners said it would "do nothing to solve the longer-term crisis".
'Rain still needed'
Graham Warren, a spokesman for the Campaign to Protect Rural England, said: "Southern needs a credible water-resource strategy for the coming decades, and at the moment it doesn't have one.
"Kent faces 120,000 extra homes under the South East Plan, yet the current resources cannot even satisfy existing levels of demand, unless propped up by emergency drought permits and mid-winter hosepipe bans."
Southern Water's water planning and strategy manager, Meyrick Gough, said: "Bewl is a key resource.
"It is vital we get it as full as possible and we envisage that the level should be raised to about 90% by the end of March.
"We still require more rain to help replenish our underground sources, because although these sources are not as visible, they are suffering as much as Bewl."
Bewl Water reservoir reached its lowest levels ever in 2005.