Anglian Water has said materials to repair the large hole in Earl Street are taking longer than it predicted.
The water company said the street will therefore remain closed over this coming weekend as it awaits made-to-order fittings.
A spokeswoman said: “The fittings need to be produced and sent to site to fully repair the storm drain that was damaged.
“We expect the fittings to arrive tomorrow (Thursday) and the repair to be completed on the same day but this has meant the reinstatement of the road will start later than originally planned.
Anglian Water said it had agreed with the Highways team at Northamptonshire County Council that the water company will do a “permanent reinstatement” of the carriageway.
The spokeswoman added: “Although this takes a little longer, it means we won’t need to return at a later date.
“We’re therefore expecting Earl Street to be open by the end of next week (Friday May 8).
“We’re sorry for any inconvenience caused.”