Scotland Yard officers have said they believe allegations that a ring of prominent politicians and members of the establishment abused and terrorised children as young as seven more than 30 years ago and went on to kill three young boys.

Detectives appealed for victims and witnesses to come forward and identified a flat in Dolphin Square, London, near the Houses of Parliament, as a scene of some of the alleged abuse, as well as military premises and other locations across London and the home counties.

So far one victim, known by the pseudonym Nick, has come forward to tell of a decade of abuse he suffered at the hands of people including senior politicians and members of Britain’s establishment, and of three homicides. Police as yet have no bodies, full names of those abused or killed, or exact locations where the killings took place.

But the detective in charge of the investigation pointedly described Nick’s allegations as “true” and said Nick had been abused from 1975 to 1984, between the ages of seven and 16.

Now in middle age, Nick has given partial names of other children who were abused, the Guardian understands, and has given names of “VIPs” alleged to be involved in the abuse. He is understood to have been scared of reprisals for telling detectives about the things powerful people did to him and other children.

If the allegations are correct, it represents one of the worst scandals in modern British history and endangers already thin public trust in the politicians who govern the country.

Police promised on Thursday to investigate “without fear or favour” but declined to say if any of those named by Nick had been interviewed as witnesses or suspects.

Detectives announced last month that they had launched homicide inquiries and said they were scouring records of missing children from over three decades ago to identify those who may have been killed.

Detective Superintendent Kenny McDonald, in charge of the investigation, said police policy dictates that officers believe a victim unless evidence emerges to undermine their account, but in Nick’s case experienced detectives from two teams had concluded his accounts were true. “Nick has been spoken to by experienced officers from the child abuse team and experienced officers from the murder investigation team. They and I believe what Nick is saying is credible and true.”

Nick waited 30 years to come forward and talk to detectives, having talked to the media first. It is clear detectives are not just investigating but building criminal cases to take to court.

Deputy assistant commissioner Steve Rodhouse thanked the media for their work but warned them not to compromise crucial witnesses. He said in one case Nick had been shown a picture of a suspect by a reporter. “I need to be able to convince a court that any identification made by Nick was done within the rules … and Nick was, crucially, identifying the [person] he remembers from 30 odd years ago, rather than the photograph he was shown by journalists in the more recent past.”

McDonald said: “I appeal to men who were subjected to abuse 30 years ago to come forward. We are also investigating the murder of three young boys – we are determined to find answers.”

He said people who lived at or visited Dolphin Square in the 1970s “will have seen or heard something that they only understand the significance of now.

“I would ask you to trust me. I will support you, and do everything in my power to find those responsible and bring them to justice. I need your accounts to help me do that. The abuse he has detailed that he was subjected to was carried out by a man on his own, a group of men or during what have been described as parties.”

Nick told the BBC last month that the abusers would inflict brutal punishments on any child who did not obey orders and children were picked up in cars to be taken to locations where they were attacked.

“People who drove us around could come forward. Staff in some of the locations could come forward. There are so many people who must have had suspicions. We weren’t smuggled in under a blanket through the back door. It was done openly and people must have questioned that and they need to come forward.”

Police said Scotland Yard officers had spoken to the family of Martin Allen, who disappeared in 1979 at the age of 15, but were not yet linking his case to the VIP abuse ring investigation.

Rodhouse said no evidence had been uncovered of police being ordered to drop inquiries into abuse at children’s homes, some of which was allegedly carried out by senior figures.

One such allegation came from the highly respected former senior detective Clive Discoll, who has said he was ordered to shut down an investigation into allegations in a children’s home in Lambeth, south London.

Police said their inquiries into claims that prominent people abused children and the cases may have been “overlooked” or covered up now spanned 18 separate investigations, including one into the Elm Guest House in London.

The police inquiries followed allegations of abuse involving senior politicians and high-profile figures made by the Labour MP Tom Watson in the Commons in a question to the prime minister. Watson said on Thursday: “Nick was frightened of these powerful people and will be relieved the police have publicly believe his allegations against very powerful people.”