The A23 has been closed between the Fleming Way and Lowfield Heath roundabouts.
A spokesman for Thames Water said the road was expected to remain closed until the end of May.
Speaking today (Thursday May 8), the spokesman said the closure was to allow sewer pipes to be laid under the road.
The southbound carriageway will reopen after May.
Engineers hoped the road will fully reopen by late June.
Mark Taylor, Thames Water’s head of programme delivery, said: “We’re sorry for the ongoing disruption our work is causing, particularly now that we’ve fully closed this particular stretch of road.
“The sewer network was is urgent need of attention and we’re working hard to make sure the upgrade is finished as quickly as possible.”