Northants County Council says it is seeking national funding to help pay for repairs to the A6 Rothwell-Desborough bypass.
The road has been closed since mid-January after an inspection found crumbling foundations where it crosses the River Ise.
It was expected to re-open next week but after further investigations, the closure has been extended to the end of the B576 until at least the end of April.
The county council says it doesn’t know how much the works will cost but its current focus is on getting it done.
A council spokesman said: “This section of the A6, which was originally part of the national trunk road network, was built by the Highways Agency and opened in 2003.
“Shortly after this it became the responsibility of the county council, when it was ‘de-trunked’.
“As has been widely reported a routine highways inspection last month discovered damage to the road where it crosses the River Ise, prompting an immediate closure.
“Further investigation work has discovered further damage to an adjoining section of the A6, so the closure has been extended to the B576 until at least the end of April.
“Currently all focus is on getting the work done so that the road can be opened as soon as possible.
“It is not yet known how much the work will cost and this will not be known until the work has been completed.
“However, we are working with all associated parties and are identifying any funding available nationally for isolated situations such as this.”