The opposition SNP group on Moray Council has written to the Scottish Commissioner, Kaliani Lyle, and the Scottish Government's Minister for Culture, Linda Fabiani, about the closure of seven of the 15 libraries.
This was agreed despite a Equalities Impact Assessment from council officials recommending three be saved due to the impact on elderly, disabled and families with young children.
SNP education spokesman Councillor Mike Shand asked the commissioner and the minister if the decision of the Conservative/Independent coalition is breaking the Equalities Act 2010.
He said: "The SNP has very grave concerns the decision forced through by the Conservative/Independent Administration is illegal in terms of the Equalities Act and could leave the council open to legal challenge. This would have a significant cost implication for the council, negating any of the savings they hope to achieve."
The council has a cuts target of £30m over three years. The decision to close Dufftown, Cullen, Burghead, Findochty, Hopeman, Portknockie and Rothes libraries and to withdraw one of two mobile library vans is part of a £357,000 saving in the libraries service.
Conservative Council leader Councillor Allan Wright said the eight remaining libraries were adequate for the needs of the people of Moray, given the "frightening" financial pressures facing the council.