The New York Times, citing a leaked copy of the document to be released Friday, reported that it found Israel used "excessive and unreasonable force" after meeting "violent resistance" from some of the passengers.
Israel and Turkey have been in dispute over an apology for the May 31, 2010 raid in which nine Turkish activists were killed.
Former New Zealand prime minister Geoffrey Palmer led the UN-mandated investigation into the raid on the flotilla that was attempting to take aid to the Gaza Strip through an Israeli blockade.
The release of the report has been delayed several times this year. Turkey has demanded an apology for the deaths, Israel has refused and there had been no agreement on the final version of the report.