Roadworks on the A47 at Lynn have been hit by recent poor weather, officials say.
Highways Agency contractors expect to begin the second phase of the project to repair three bridges between the Pullover and Hardwick roundabouts later this week.
But project manager Ranjit Mistry has confirmed the project is now more than a week behind schedule.
He said: “The next phase of the works has been delayed because of the recent bad weather and we have also needed to complete additional work on the bridges.
“We are endeavouring to finish the scheme by end of July, but this is weather permitting.”
The route will be closed in both directions between 8pm on Thursday and 6am on Friday so the contraflow system which is currently in place on the eastbound carriageway can be moved to the westbound side.”
The agency, which has thanked drivers for their patience, said motorists should allow extra time for their journeys.