"This intervention is illegal," he told an audience in Winnipeg on Tuesday.
He believes it also sets a dangerous example. "Other countries may use this argument in the future to intervene on the basis of this precedent."
Boutros-Ghali, the Secretary General of the United Nations from 1992 to the end of 1996, fears the diplomatic struggle, and the failure to reach a consensus in the Security Council, will weaken the UN's ability to play a central role in such future crises.
"I still hope that the UN will be able to overcome this crisis and that the UN will continue to play a role, even if this role is very marginal," he said.