A report on a Metropolitan Police investigation into an alleged group of paedophile VIPs who abused and killed children in the 1970s and 1980s has identified 43 failings.
The decision to abandon Operation Midland should have been taken "much earlier", author Sir Richard Henriques said.
Henriques found the operation was "riddled with errors" - officers misled a judge into granting search warrants and should not have identified suspects while "publicly stating the allegations were credible and true", he added.
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Scotland Yard's investigation into claims by a lone complainant identified only as "Nick" closed without a single arrest in March. Among those publicly named in connection with the investigation were former MP Harvey Proctor, former Army chief Lord Bramall and the late Tory peer Leon Brittan.
Met Police Commissioner Sir Bernard Hogan-Howe said he accepts "accountability for these failures" and apologised to those involved.
"It is a matter of professional and personal dismay that the suspects in the investigation were pursued for so long when it could have been concluded much earlier," he said.
"I am today issuing a public apology to Lord Bramall, Lady Brittan and Harvey Proctor for the intrusion into their homes and the impact of Operation Midland on their lives."
Proctor said he accepted Hogan-Howe's apology but lamented the "irreversible" impact on his life and that of his friends and family. He also said it was "completely outrageous" that Scotland Yard had published the findings of the report on the day of the US election.
"They have chosen this date deliberately... as part of their PR campaign to cover up the disaster that was Operation Midland," he said.
While Hogan-Howe backed the report's findings, others in British policing and victims' groups appeared to oppose it, saying it could damage the fight to bring sexual abusers to justice, reports The Guardian.
An NSPCC spokeswoman said: "It takes unimaginable bravery for victims to come forward – we are concerned that this report's recommendations could undermine the confidence of victims to do so."
David Tucker, the crime lead at the College of Policing, said: "To start an investigation from a position of doubt is unlikely to encourage victims to come forward."
Journalist Mark Watts, who covered the investigation for the BBC, went further, sending out a series of tweets in which he said the "conclusions about how police should investigate child sexual abuse will set Britain back decades".
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