Sheffield Council is being reported after failing to release details of private organisations bidding to run libraries.
The authority said the need to protect its confidentiality when negotiating with third parties outweighed the public interest in making the details public – or revealing which libraries could close.
The council said it may have to close up to 14 of Sheffield’s 27 community libraries to save £1.6 million a year as part of budget cuts, unless third parties are willing to take branches over.
Opposition Liberal Democrats are now reporting the Labour-run authority to the Information Commissioner for refusing The Star’s requests.
The council also took a month longer than the 20 days in which it is supposed to respond to such requests.
Coun Shaffaq Mohammed, council Liberal Democrat group leader, said: “It’s a scandal Sheffield’s Labour councillors have been discussing which libraries to close behind closed doors for months, but haven’t got the courage to come clean with the people of Sheffield.
“Enough is enough and we will be referring the council to the Information Commissioner over their non-compliance with these reasonable requests.”
A spokesman for Deputy Prime Minister Nick Clegg, Liberal Democrat MP for Sheffield Hallam, said: “It is appalling Labour is keeping details in the public interest from public scrutiny.
“People have a right to know about the future of cherished facilities.”
Concerns were echoed by the Green Party.
Coun Rob Murphy, council Green member for Central, said: “The council should release the names of third parties and also extend the consultation period about the future of libraries.
“We would also make a call for the council to open the books of the library service, so any groups wishing to take over can scrutinise the finances accurately.
“Community groups in Stocksbridge wanting to take over the leisure centre, and in Highfield and Burngreave interested in running adventure playgrounds, ended up frustrated because they were not given access to such information – which is necessary to make an informed decision.”
The council’s rejection of The Star’s request was also criticised by Grenville Wilkinson, of Walkley Forum, which is campaigning to save its area’s library.
He said: “It is not acceptable at all to keep this information behind closed doors. The council is failing to act in an open or transparent manner, which I feel is wrong.”
Coun Julie Dore, council leader, said it would publish the names of third parties wanting to run libraries, and branches which could close, next month.