Scotland Yard has been condemned for its handling of a multimillion-pound investigation into an alleged VIP paedophile ring in Westminster after the former nurse at the centre of the allegations was found guilty of fabricating his extraordinary claims.

Carl Beech, 51, had alleged that he was among the victims of an establishment group – including high-ranking politicians and military figures – who he said kidnapped, raped and murdered boys in the 1970s and 1980s. His allegations triggered a Metropolitan police investigation that ended without an arrest being made.

But on Monday Beech, known by the pseudonym “Nick”, was found guilty at Newcastle crown court of 12 counts of perverting the course of justice and one count of fraud relating to a £22,000 criminal compensation payout.

Beech, who now faces a lengthy jail term when he is sentenced on Friday, stood impassively in the dock as the verdicts were read out. A cry of “Yes” rang out from the public gallery.

During the 10-week trial, Beech stood by his claims, giving a graphic account of the three child murders he alleged to have witnessed, insisting: “I was there and I know it took place.”

But the jury’s decision, reached after four and a half hours’ deliberation, brings an end to a case which was described by the Crown Prosecution Service as “quite unlike any other”. The trial heard that Beech fled to a remotepart of northern Sweden whenhis story began to unravel.

His conviction comes as the Guardian can reveal:

Beech gave talks to children as young as five about the dangers of abuse on behalf of the NSPCC.

The journalist who led reporting of the false claims has admitted to repeated doubts about his key source and said he was taken in by Beech’s middle-class credentials.

Beech helped start a charity exhibition highlighting child abuse survivors’ stories which went on display at City Hall in London.

Beech – who pleaded guilty to child abuse image offences in a separate trial – told the Met in 2014 that his stepfather, Maj Raymond Beech, had begun ferrying him to parties when he was seven where he was sexually abused by a group of high-profile men.

His claims, pushed by the Exaro news agency, were splashed across the front pages of national newspapers. The Labour deputy leader, Tom Watson, met Beech to talk about his allegations and the former health worker told the trial that Watson was part of a “little group” supporting him.

In December 2014, the officer leading the £2m Operation Midland, Det Supt Kenny McDonald, described Beech’s account as “credible and true” on TV news bulletins. McDonald retired before the trial.

Beech claimed the gang of abusers included the former prime minister Edward Heath, the former home secretary Leon Brittan, the former Tory MP Harvey Proctor and the D-day veteran Lord Bramall. Others accused of abuse were the former head of MI6 Sir Maurice Oldfield, the Labour peer Greville Janner and the former head of MI5 Sir Michael Hanley.

On Monday, Bramall, 95, hit out at the Met’s “incompetence”. The former head of the British army told the Times: “It was Carl Beech’s outrageous and totally untrue allegations which perverted the course of justice. However, the incompetence of the Metropolitan police service, in the improper way that they accepted Carl Beech’s allegations at their face value, lent them unwarranted credibility.”

Facebook Twitter Pinterest The former Tory MP Harvey Proctor. Photograph: Owen Humphreys/PA

Proctor – who gave testimony during the trial – criticised Beech, Scotland Yard, Watson, Exaro and the BBC for its initial reporting.

He said: “The Metropolitan police were lapdogs to Mr Watson’s crude dog whistle. It’s time for the torchlight to take a closer look at Mr Watson. It is now beyond doubt that all of these allegations could never have been true and only someone with spectacular bad judgment could think that they might be.

“Mr Watson likes a stage to call upon others to apologise or to put right past wrongs. It is time for an apology from him to me and everyone named or implicated in this truly disgraceful chapter in the history of British policing which was allowed to happen because of internal failings at the highest level within the Metropolitan police.”

Proctor, who is suing the force for compensation, added: “When resources are in short supply, the Met police should apologise to the taxpayer for squandering millions of pounds - an estimated £4m directly and indirectly – by their malice, incompetence and negligence on Operation Midland.”

He called for urgent inquiries, including safeguarding investigations, into places where Beech has worked in the past, including the NHS and Care Quality Commission.

Watson defended himself, claiming it was not his role to judge whether Beech was telling the truth and clarified he only met Beech on one occasion in July 2014, several months before Scotland Yard launched its investigation.

“During that meeting, Nick said very little and did not name any of his alleged abusers,” the MP said. “I reassured Nick that the police had made clear that all allegations of historic sex abuse would be taken seriously and treated sensitively. That is what the police had asked me to do and it was the process I followed with all those who claimed to be survivors of historic child sex abuse.

“It was not my role to judge whether victims’ stories were true. I encouraged every person that came to me to take their story to the police and that is what I did with Nick. I hope this trial, and the case of one person, does not prevent survivors of child sexual abuse coming forward and reporting their experiences to the police.”

Responding to Proctor’s criticisms, which included that of the Labour MP’s speech to the Commons in 2012 in which he said there was “clear intelligence suggesting a powerful paedophile network linked to parliament and No 10”, Watson said: “I believe that my question to the prime minister in October 2012 surfaced the issue of historic child sexual abuse in a way that meant it could not be ignored as it had been. My question led to the recovery of police files and the conviction of three men for child sexual abuse.”

Beech alleged the offences had taken place in an array of locations in London – such as Dolphin Square, a private members’ club, the Carlton Club, Elm Guest House and London zoo – as well as military bases and on Heath’s yacht.

Beech, who was a paediatric nurse at the start of his career, pleaded guilty in a separate trial in January to four counts of making indecent photographs of children, one count of possessing indecent images of children and one count of voyeurism.

After Monday’s verdict, the policing watchdog, the Independent Office for Police Conduct, announced it had cleared three officers involved in Operation Midland following an investigation into how a district judge granted police warrants to raid the homes of those Beech accused.

Responding to Beech’s conviction, the deputy Met commissioner, Sir Stephen House, said: “It remains true that investigating allegations of sexual offences is a very complex and challenging area of police work. Those complexities are compounded where those allegations stretch back many decades, as was the case in Operation Midland.

“Additionally, many of the issues relating to the investigation of non-recent sexual offences continue to be examined by the independent inquiry into child sexual abuse, with which the Met is fully cooperating, and it would be inappropriate for us to pre-empt the inquiry’s findings with any wider commentary on this area of policing at this time.”