The road which runs through the Bondgate Tower, in Alnwick, will be closed until next week after an inspection found more serious damage than previously thought.
The tower was closed this morning for the stone to be examined after a recent car accident.
But the inspection found that there was more damage caused than previously thought.
It will be closed until at least Wednesday for emergency repairs to be made.
Heading into Alnwick, from the south, diversions are in place along Hotspur Street and Green Batt.
However, in the town centre, signage for the closure has been criticised after motorists trying to head out of town were having to make u-turns on Bondgate Within.