A seven-mile section of the A6 just south-east of Market Harborough will be shut until the end of February, because of a faulty bridge.
Northamptonshire Highways closed the road on January 18, when an inspection revealed the bridge weakness.
This week they made the decision to keep the road closed for another five weeks for major repairs to be undertaken.
A spokesman for Northamptonshire Highways said: “Northamptonshire Highways has taken the decision to keep the A6 closed.
“This is following a review which has shown the issues to be more significant, and the condition of the road is a safety risk.”
The closed section of the A6 begins about two miles south-east of Market Harborough, at the B576 junction to Desborough.
The road is closed in both directions right down to the A14 junction near Rothwell - a seven mile stretch of A-road that bypasses Desborough and Rothwell.
The Highways spokesman said: “We are now starting works with all our partners and have specialist teams on site and plan to re-open the road at the end of February.
“The works we are undertaking will also be affected by adverse weather.
“All roadusers are advised to avoid the area or follow the signed alternative route.”
The diversion takes traffic from the A6 along the A427 towards Corby, then along the A6003 to its termination at the A43 roundabout, and then along the A43 to the A14.
It turns the seven miles direct route along the A6 into a 22-mile diversion.
The Highways spokesman said: “We would like to apologise for this extended closure and the inconvenience this is causing.
“This route must remain closed until we feel it is safe to reopen.”