The Council of Europe found that a "catalogue of errors" by Nato and coastguards resulted in the deaths of 63 refugees sailing in a small boat from Libya last May.
Only nine people on board survived after the craft drifted in open seas for two weeks. Two Nato ships nearby failed to respond to distress calls, and no country launched a rescue mission, the council said. It concluded there were many opportunities to rescue the survivors. Nato says it has no record of contact with the boat.