Parts of the South and South East face forecasts of 34C or 35C on Monday and Tuesday after the all-time hottest May temperature was recorded at 33.5C in Heathrow, west London, shortly after 1pm.
South East Water apologised after it had to hand out bottled water to some of its customers in Kent and Sussex.
A total of 250 properties in three Kent villages were suffering intermittent water outages or low pressure on Monday.
A burst water main in Eastbourne, East Sussex, affected 168 properties, while 64 customers suffered from issues with booster pumps at Radfall near Broomfield Gate in Whitstable, Kent.
There were also “ongoing water supply problems” for fewer than 20 properties on higher ground in the Ulcombe area of Kent.
South East Water incident manager Steve Benton said in a statement on Monday: “We’re sorry to all impacted customers for the disruption caused.
“We know water supply issues during hot weather are deeply frustrating and incredibly inconvenient and we are working as quickly and safely as possible to resolve them.”
He added: “As a result of the recent hot weather, we’re seeing increased demand across our network and we’re having to pump far more drinking water than usual.
“We’re working hard to maintain supplies to customers across our network as the hot weather and increased demand for drinking water continues.
“Some customers on higher ground may, however, experience low water pressure or intermittent supply at peak times.”
South East Water customers also faced major disruption for more than a week in January.
At its height, the company said more than 30,000 customers across Kent and Sussex were without water or experiencing low pressure after Storm Goretti caused burst pipes and power cuts.
The Kent villages affected by the current low pressure and outages are Charing, Challock and Molash.
A technical failure at a pumping station near Charing meant the company “struggled to push water to properties on higher ground”.
It has opened a bottled water station at Challock village hall.
A technician was working on the booster pump issue in Radfall and the company delivered bottled water to those on its “priority services register.”
Staff were working to repair the burst water main in the Decoy Drive area of Eastbourne.
