Major roadworks are set to begin on the A487 near Newport next month.
The project, which involves replacing a crucial drainage structure beneath the road, is due to start on January 6 and will last for eight weeks.
The existing structure could not be repaired, making a full replacement necessary.
The work aims to ensure the long-term resilience of the road, maintain vital links between communities, and protect against climate change impacts.
Temporary traffic signals will manage a local diversion route.
However, heavy goods vehicles will need to follow a longer diversion, about 35 miles.
A spokesperson for the Welsh Government said: “This work is essential in order to ensure the road can remain open in the years to come.
“We understand this will cause disruption in the short term while the work is under way, and we thank drivers for their patience.
“It is vital this work is carried out at this time for the future resilience of the road.”
The government has confirmed that consultations have been held to find the least disruptive time of year for the works.
Support from Business Wales will be offered to businesses affected by the road closures.