A report into the deaths of 63 Libyan refugees on a small boat last May said a "catalogue of errors" by coastguards and Nato meant they were never rescued.
The Council of Europe inquiry says two Nato ships close by failed to respond to distress calls, and no country launched a search and rescue mission.
Only nine people survived the 15-day Mediterranean voyage after the boat ran out of fuel and drifted back to Libya.
Nato has said it has no record of any aircraft or ship contacting the boat.
However, it says other search and rescue missions were launched around this time.
The report says some 1,500 people died at sea trying to reach Europe in 2011.
Many of these were trying to flee from Libya or other conflicts in North Africa.