The council said it had concerns about releasing more details
A council has been told it breached the Freedom of Information Act by failing to provide an AM details about the cost of employing education consultants.
Leanne Wood says the Vale of Glamorgan council must end its "secrecy" after the information commissioner's ruling.
The authority spent £46,000 on two consultants but did not say how many days they worked or what each was paid.
The council said it gave a breakdown of work and total costs, but had "grave concerns" about releasing more details.
The council has been given 35 days to provide the information or face High Court action.
The commissioner, Anne Jones, ruled on an appeal about information on the employment of consultants Dewi Jones and Celia Butler submitted by Ms Wood, a Plaid Cymru South Wales Central AM.
The commissioner said she had not been provided with any firm arguments to support the council's view that disclosure of the information would prejudice the interests of the consultants or the council.
Ms Jones said: "There is a strong public interest in transparency in relation to the use of public money and ensuring that public authorities are achieving the best price for work... outsourced to external companies."
The commissioner ruled that the council breached and incorrectly applied six sections of the Freedom of Information Act, including failing to provide the information within 20 days.
Ms Wood said she hoped the council would provide the information requested as a matter of urgency and "end the cloak of secrecy".
She said: "People have a right to know how much of their money the authority is paying for these consultancy services - was it £500, £750 or £1,000 a day?
"At best it shows the council did not have a grasp of the Freedom of Information Act. But at worst it will be seen as an attempt by the council to cover up what it paid consultants for a day's work."
Peter Evans, the council's director of legal, public protection and housing, said Ms Wood was provided with a breakdown of the work undertaken by the consultants and the total cost.
He said this demonstrated the authority's commitment to transparency and public accountability as it allowed Ms Wood to scrutinise the costs against the work undertaken.
Mr Evans added: "We had grave concerns with regard to release of further information.
"One of the consultants had written to us to say that they considered the release would breach the Data Protection Act and the bills submitted by the other consultant contained personal details including his address, bank account number and sort code.
"In the circumstances the council would not have released further information without the authority of the information commissioner."
Mr Evans said that since the decision notice the commissioner's office had confirmed that the bank details should not be released.
He said the council would now inform the consultants of the decision notice and the appeals process with regard to the remaining information.
Mr Evans said: "The Freedom of Information Act has been in operation for nearly five-and-a-half years and in that time only four decisions, out of more than 1,500 requests received, have been made against the council by the commissioner."