The Home Office-commissioned investigation, conducted by former head of Child Exploitation and Online Protection Centre Jim Gamble, found the overwhelming number of U.K. agencies involved in the search for Madeleine damaged Portuguese police.
Then-3-year-old Madeleine disappeared in May 2007 while vacationing with her family in Praia da Luz in Portugal. The first investigation into her disappearance was closed in July 2008 with little trace of the missing child, and the inquiry was reopened in 2013.
Gamble's report found the CEOP, the Metropolitan Police, the Serious Organised Crime Agency and the National Police Improvement Agency had all intervened within weeks, creating "frustration" and "resentment" among the Portuguese authorities working the case.
The unpublished report, whose details were shared with Sky News, found the decision to put police from the McCanns' home of Leicestershire in charge of the investigation was a mistake, as the force was not equipped to handle the demands of such a significant investigation.
"All of us... your first gut reaction is you want to help...so everyone came with best intention, that created a sense of chaos and a sense of competition...and in many instances in my opinion wanting to be seen to help," Gamble told Sky.
"It was unhelpful...I've no doubt relationships from the outset with the Portuguese were impacted by it and I think that had a long-term negative effect on the investigation."
Neither were the Portuguese authorities spared from Gamble's criticism. He said the initial inquiry was "haphazard" and potentially crucial leads went without follow-up.
If Madeleine is still alive, she would be 11 years old.