The pipe under the A2047 London Road in Portsmouth ruptured early on Thursday flooding the road and leaving homes without water supplies.
Portsmouth Water said the road had been left "structurally unsound" as well as covered in silt and mud.
The firm said the water supply had been fully restored and the road would partially reopen on Friday.
Distribution engineer Paul Treagust said: "At approximately 00:30 this morning the large 18-inch main burst between Montague Road and Stubbington Avenue.
"Our staff were on the scene straightaway and... there has been no risk to water quality as a result of the incident."
Portsmouth Water spokesman Ian Limb said: "It's a huge beast of a water main, it's destroyed the road."
The company said London Road was expected to open at 06:00 on Friday using one lane controlled by traffic lights.
A busy road closed by a water main burst may not fully reopen for another four days, a water company has said.