The council hopes the £116m busway project will be completed on budget
The contractor building a £116m guided busway in Cambridgeshire fears the costs will over-run.
The 16-mile (25km) busway, which links Huntingdon, St Ives and Cambridge, is due to be completed by next April.
A report to Cambridgeshire County Council says contractor BAM Nuttall has reported it is expecting the project cost to exceed its original estimate.
The council said it still hoped the project could be completed on budget, but was in talks with BAM Nuttall.
Matt Bradney, the Tory council's cabinet member for transport, said: "When the scheme was first drawn up we planned for this both in terms of our budget and having a robust contract that puts almost all the onus on the contractor to come in on budget.
"Within the £116m budget some £5m was set aside to cover risks such as this.
"However, it is sensible that we take precautions and therefore non-essential aspects of the scheme (such as park and ride buildings) will be delivered after buses are running.
"We and BAM Nuttall continue to work closely on this and works are progressing well to build the guideway which will see buses using it next year."
But the busway has been criticised by the Liberal Democrat opposition group on the council.
Liberal Democrat leader on the council David Jenkins said: "Nothing can conceal the fact that it was an unpopular project to start with and that it was squeezed into a financial straight jacket.
"We are now seeing the impact on its schedule, its cost and on how much the county council is going to have to put in to fund it.
"Lib Dems have consistently advised the council firstly not to build the busway and secondly to be more conservative with its financing.
"Now we are all suffering because the ruling Conservatives did not follow our advice."