Four years after it fully opened, council taxpayers will pick up an extra multi-million pound bill for the A1073 Spalding to Eye Improvement, now known as the A16.
A report to go before county councillors this week recommends another £4.160million is taken from budgets to pay additional costs because the council underestimated the true value of some of the land it bought to build the road.
Other compensation claims are understood to be in the pipeline and could total half-a-million.
The road has already cost £90.8million with the Government contributing almost £70 million and Lincolnshire County Council picking up the tab for the remainder, apart from some minor works.
Labour county councillor Phil Dilks said an earlier report gave the overspend as £4.618 million – including a provision for the future claims.
He said: “It is essential in the public interest that these extra costs are brought out into the open and properly explained and accounted for, particularly given the history and controversy of the project.
“It was this scheme which led to Jim Speechley, the former Crowland councillor and Conservative leader of Lincolnshire County Council being sent to prison for abuse of office, resulting in Lincolnshire being dragged through the mire and regarded nationally as a basket case among local authorities.
“As a county councillor at the time, I was lobbied and met campaigners for an alternative route which was rejected on the ground that it was predicted to cost £2million more than the preferred route.
“Given that the actual costs are already pushing £5million more than forecast, the people of Lincolnshire deserve a proper explanation of how the county council has ended up in what appears to be another fine mess.
“The Tories who run Lincolnshire are threatening to close more than 30 libraries yet appear unconcerned at a £5million overspend on the A1073.
“£5million would keep every library in Lincolnshire open for at least the next two-and-a-half years.”
A council report reveals some of the land it acquired was estimated at agricultural value but then found to have some development “hope” because it was close to Peterborough.
The report continues: “The value of this land was the subject of a decision by the Lands Tribunal. The value of compensation payments paid to affected landowners is higher than originally estimated.”