It comes as an issue at Testwood Water Treatment Works led to an automated shutdown of the reservoir on Tuesday due to water quality concerns.
It had to be emptied and refilled before supply could resume – with thousands of properties, several schools and Southampton General Hospital and Princess Anne Hospital hit by outages.
In Southampton six schools – The Cedar School, Sinclair Primary and Nursery School, Fairisle Junior School, Fairisle Infant and Nursery School, Hollybrook Junior School, and Hollybrook Infant School – were fully closed due to utilities failure.
Tomorrow five Southampton schools, including Compass School, The Cedar School, Fairisle Infant and Nursery School, Sinclair Primary and Nursery School, and Fairisle Junior School, are scheduled to close fully for the same reason.
Calmore Junior School and Calmore Infant School will be closed fully. Halterworth Primary School in Romsey is due to open at 11am.
A message to parents added: “If it is not possible for school to open at 11 am, we will contact parents by 10am.”
Southern Water said they have supplied dedicated tankers to Southampton General Hospital and Princess Anne Hospital, which are also affected by the issue.
The company apologised for the “ongoing disruption” and said on its website it hopes supply will be restored throughout Thursday, but said the issue may not be fully resolved until the weekend.
“The fix at Testwood supply works is ongoing and we hope to complete the initial phase by tomorrow morning,” the firm said.
“We then need to re-charge our water network and restore supply to homes that were affected. If these works proceed to our current plan, customers should start to see supplies restored during tomorrow, but some areas may take longer.
“We are working towards all customers being back in supply by the weekend.”