The A6 Rothwell Desborough bypass will remain closed until at least the end of April.
The stretch between Junction 3 of the A14 and B576 has been closed since January 18 after a routine inspection uncovered foundation failures with the bridge crossing the River Ise.
It was scheduled to re-open by the end of February - but it’s now been put back a further two months after further issues were found with the approach to the railway bridge.
A spokesman for Northamptonshire County Council said: “The works we are carrying out have shown the importance of our routine highway inspections.
“Without these the issues would not have been uncovered.
“By finding these early we are able to undertake the repairs to the foundations to ensure the road remains safe and prevent further deterioration.
“We apologise for the extended time period of the closure but we believe the best thing to do is to address all issues on the bypass under one road closure.
“This way we can be confident we will not need to revisit the location again.
“We’d like to thank road users and residents for their continued patience while we complete our essential work and reassure everyone that we’ll be doing everything we can to complete this work as quickly and safely as possible.”
The spokesman added that as investigations continue there is a possibility the delay could go on further.
From the north travelling south on the A6 drivers are asked to:
• Divert on to the A427 towards Corby;
• Traffic will continue along the A427 exiting at the A6003 roundabout.
• Traffic will continue along the A6003 (Uppingham Road) to its termination at the A43 roundabout.
• Traffic must then access the A43 (Southbound towards Kettering) and continue to junction 7 where it will leave the A43 and join the A14 travelling westbound towards Desborough and Rothwell.
• Traffic will then exit on to the A14 at junction 3.