Police have condemned the BBC for an edition of Panorama that cast doubt on claims a VIP paedophile ring was active in Westminster.
In the programme, broadcast on Tuesday, a man identified only as "David" said he may have been encouraged to make his allegations by campaigners fighting to expose historic child sex abuse.
He told Panorama that he gave the names of politicians involved in abuse, including former home secretary Leon Brittan, who died in January after the accusations were made, "as a joke suggestion to start with".
The ring is alleged to have murdered three boys in the 1970s and 1980s and will be the subject of a judge-led inquiry.
The Met previously referred to claims by another alleged victim, known as "Nick", as "credible" when it appealed for him to come forward. It later said it should not have used the word as it implied it was pre-judging his evidence.
Scotland Yard said it had "serious concerns" about the Panorama broadcast and the impact it could have on its ongoing investigation into alleged sex abuse and murder.
The force said in a statement: "We have warned previously about the risks of media investigations compromising a criminal investigation.
"When we initially launched our Operation Midland appeal, we specifically highlighted how a media organisation - the BBC in fact - had shown pictures of individuals to 'Nick' which could compromise the evidential chain should a case ever proceed to court.
"We continue to be concerned about approaches to witnesses by all media, and that warning was reinforced by the Attorney General."
The police statement added: "We recognise that there is a public interest in reporting and commenting on the police and our investigations. We can and do accept criticism of our policing operations.
"But we do believe there is a distinction to be made between fair comment and impacting on victims and witnesses in a way that may damage them or a criminal investigation."
Panorama editor Ceri Thomas said they were "enormously concerned" about producing something that would deter victims.
She added: "But we think we have to be able to scrutinise how the police go about big investigations like this and there really isn't a way to do that without looking at the way that police have treated statements from victims and treated the evidence they've brought forward."
A BBC spokesman said the programme was an "important and fair" piece of investigative journalism.
He said: "[It] rightly asks legitimate questions about the conduct of the police, journalists, campaigners, and politicians in handling historic allegations of child abuse.
"We were aware the Met Police has concerns about this Panorama going ahead but as they recognise there is public interest in reporting on their investigations."