Cambridgeshire County Council has had to plug a £14 million shortfall for the guided bus after banking on developer money from the Northstowe new town which never came.
The Tory-run administration has had to borrow the money after costing the project by taking into account Section 106 income - money paid to the council by developers of new homes to provide community facilities.
But work on the new town failed to start and the council was left with a gaping hole in its finances.
At the time the business case for the guided bus was prepared, it was expected the £14 million would be received by the end of the project's construction phase.
It was an expensive gamble for the county council, however, which has had to take an additional loan to cover the shortfall.
The initial costings for the project were not broken down and the figure only came to light following a request from Liberal Democrat Highways Spokesman, Kilian Bourke.
Now he has called on the Tories to give taxpayers a detailed breakdown of borrowing on the guided bus "in the interests of transparency".
He said: "The Tories costed this project with money they hadn't got but hoped they would receive from the Northstowe development; this was a gamble and it didn't pay off.
"Now the taxpayers have been left picking up the bill for the interest on this loan. The guided bus is costing more and more at every turn and it is time the Tories gave us a full breakdown of the cost of this project in the interests of transparency.
"Conveniently for the Conservative administration, the council does not give information on the borrowing costs of individual projects, so these figures are embedded in the general borrowing requirement. But the bottom line is that the council has had to borrow £14m more to plug this gap.
"We should not have to tease out this information piece by piece. This is public money and the public have a right to be given the full picture."
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