A bridge linking Burton Latimer and Isham that has been closed since January will now not re-open until November.
The Station Road bridge, near Weetabix, shut so Network Rail could raise it as part of the electrification works with a planned opening date of September 24.
But the road will now not open until Monday, November 25, after additional gas pipe protection measures were identified during work to re-instate the new bridge deck.
A Network Rail spokesman said: “Firstly, we would like to apologise to residents and motorists in Burton Latimer, Isham and the surrounding areas for any inconvenience caused by the longer than expected closure of Station Road Bridge.
“The bridge had been closed to allow Network Rail engineers to raise its height to provide a safe distance from overhead line equipment passing beneath as part of the Midland Main Line upgrade and was due to re-open in October.
“However, during work to reinstate the new bridge deck, additional gas pipe protection measures were identified. This means the work will now take longer than planned. The road will now re-open on Monday, 25 November.
“We are sorry for this delay and we would like thank residents and drivers for their patience.”
Diverted motorists have either been forced to take the A14 or travel along the A509 towards Wellingborough before travelling on Finedon Station Road.
The delay has been criticised on social media with one claiming it looks like no progress has been made in months.
Isham traffic campaigner Graham Rait said the diversion has made the Finedon Station Road junction more dangerous.
He said: "Like HS2 these projects are never on time or on budget, so there's no change there.
"It hasn't affected us too much where we are apart from trying to get to Sainsbury's.
"But it has made the turning for Finedon Station Road a bit more dangerous because there's simply more traffic using the road."
The hold-up is the latest delayed Network Rail project in the north of the county in the past 12 months.
A similar project in Corby also over-ran by two months last year.
And businesses on a Wellingborough industrial estate have been left in limbo after bridge closure that was supposed to last six months became - after a couple of U-turns - a closure lasting more than 18 months. They have since been denied compensation.