Moray Council has voted to close seven out of its 15 libraries as part of a move to cut £30 million over the next three years.
The decision, which has sparked widespread condemnation, was made despite a 150-page report that recommended three are not shut because of the impact on the elderly and disabled.
Now the Chartered Institute of Library and Information Professionals in Scotland has warned the authority risks failing to meet its legislative obligations.
In an open letter, the institute's council writes: "CILIPS Council are deeply concerned by the decision of Moray Council to close so many libraries after dismissing the advice of its own officials, public protest, and the Equalities Impact Assessment, which highlighted that the closure would disadvantage communities affected. We hold a good public library service is essential to the well-being of communities"
The letter adds that libraries work to improve literacy by providing access to reading.
It goes on: "It is CILIPS's view that the closures will diminish the service in such a way that it may well fail to meet the requirement of an adequate strategic network delivering core provision."
Council Leader Allan Wright said: "The content of the letter is what one would expect from the professional body for librarians. I remain satisfied that the eight libraries will provide an acceptable network for the people of Moray in these challenging financial times."