Two schools and a Covid-19 testing centre have closed and thousands of people are without water after a storm cut power to a treatment works.
The outage at the Otterbourne plant has affected more than 20,000 properties in Southampton, Winchester, Owslebury and Twyford, Southern Water said.
It has installed bottled water stations in some of the areas affected.
Merryoak Covid-19 testing centre in Southampton and Netley Abbey junior and infant schools have closed.
Southern Water said the problems began late on Monday evening.
"The storm caused a power blackout at our Otterbourne Water Treatment Works which treats water then sends it to storage reservoirs," it said.
"The storage reservoir at Yew Hill has lost levels and it is the homes served by this storage reservoir which are affected."
In a further statement released early on Tuesday evening, the water company said its teams had "worked around the clock" at the treatment works to restore supplies to customers.
It added: "Supplies have returned in some areas and we expect to see the situation improve throughout this evening."
It has installed bottled water stations at East Winchester Park & Ride (St Catherine's), and Places Leisure Eastleigh.