UK Anti-Doping has been heavily criticised for failing to tell the General Medical Council that a doctor was allegedly supplying athletes with performance-enhancing drugs.
The verdict comes after an independent review into UKAD's handling of the intelligence it was given about Dr Mark Bonar by amateur cyclist Dan Stevens.
The agency was also censured for not giving Stevens, who refused to take a targeted drug test in 2014, a more lenient sentence after his co-operation.
The review's report praised UKAD's staff for their honesty and commitment to clean sport - and admitted this was a "difficult and complex" case - but was scathing about aspects of its investigation.
"It is difficult to understand why no contact was made with the GMC when that course of action was suggested on at least seven occasions either by members of UKAD or (Stevens) and his legal representatives," said the review's chairman, former Merseyside assistant chief constable Andy Ward.
He added that "as a minimum standard of investigation, a simple check" with the GMC should have been made to find out if it, as Bonar's regulatory body, had any information that could "support or negate" Stevens' claims.
Ward also referred to "confusion and lack of clarity" in how UKAD deals with dopers who want to reduce their sentences in return for "substantial assistance" and described the agency's dismissal of Stevens' request for a lesser sanction as "harsh" and "subjective".
It was that dismissal that led Stevens to take his story to the Sunday Times, which then filmed the London-based private doctor claiming he had supplied prohibited drugs to more than 150 elite athletes.
Those claims, which were published by the newspaper in April, have now been investigated by UKAD, and the agency did eventually share its findings with the GMC. Bonar has since denied any wrongdoing.
"It is clear that opportunities to gather intelligence, secure evidence and investigate Bonar have been missed," concluded Ward, who was appointed to head up the review in the immediate aftermath of the story's publication.