The opening date of a bypass to the north of Norwich is likely to be delayed following a government probe into the building contract.
Norfolk County Council agreed to hand the Northern Distributor Road contract - worth £91m in 2006 - to May Gurney.
However, the Department for Transport (DfT) said the process was illegal and has ordered the contract is offered out for road builders to bid for the work.
Norfolk County Council said it acted to reduce costs and construction times.
Adrian Gunson, cabinet member for planning and transportation, said: "If we give it to one contractor it saves time and money.
"If that contractor is one we have worked with for a number of years then it can be argued that they know what we want and we know what they are capable of.
"If on the other hand we do not give it to one contractor, then it takes time and it bound to cost more money and it delays the process."
Ian Findlater, from May Gurney, said: "We share Norfolk County Council's frustration about this but are keen to take part in the tender process in due course.
"It's a shame that the DfT's position will delay the NDR by up to two years and undoubtedly end up increasing the cost of this badly needed project."