People living in parts of Kent and Sussex are facing a ban on unattended hosepipes and sprinklers from Monday.
Water companies advise the use of old washing-up water for plants
The restrictions will affect Southern Water and Mid Kent Water customers as a result of insufficient rain and the second driest winter since 1904.
Southern Water said over the last eight months reservoir and groundwater levels had become dangerously low.
Mid Kent Water said the decision to restrict use had been taken reluctantly and further action was possible.
About 218,000 Mid Kent customers across Ashford, Maidstone, Canterbury, Herne Bay and Whitstable will be affected by the restrictions.
The Southern Water ban covers people in its Medway, Thanet and Hastings supply areas.
From Monday, a full hosepipe and sprinkler ban will also take effect in the company's Sussex coast supply area, which extends the ban already in place in the North Sussex area.
The company said although showers in April and May had helped to keep the demand for water down, they were still the sixth and seventh consecutive months of below average rainfall.
A spokesman for Southern Water warned inspectors would be patrolling the areas affected to ensure people complied with the ban.
John Crook, water operations manager, said: "Although we have probably got enough water to go through the summer the worry is another dry winter could cause serious problems."
He urged residents to conserve water as much as possible, for example by using old washing-up water for garden plants.
Mr Crook said it had been 10 years since the last ban was implemented.
This year's restrictions will only be lifted if enough rain falls in the winter.
Mid Kent Water said its priority was for customers to have a supply of clean, safe drinking water for everyday needs.
Trevor Bishop, head of regulation and customer services, said he hoped the steps taken by the company would help reduce demand on non-essential use.
"After all, a garden sprinkler uses as much water in one hour as a family of four uses in one weekend," he said.