A report into Scotland Yard’s disastrous investigation into an alleged VIP paedophile ring will reveal the lawyer who led the independent inquiry into child sexual abuse warned detectives the star witness was a probable liar, but was ignored.

The warning came in September 2015, six months before police wound up their inquiry and declared former military chief Lord Bramall, former home secretary Leon Brittan and former Conservative MP Harvey Proctor as innocent.

The revelation that police were told with such authority their key witness, known at the time only as Nick, was a fantasist, but pressed on, will heap pressure on the Metropolitan police and its leadership.

On Friday the force will publish three chapters of a report by Sir Richard Henriques that reveals 43 police errors and is expected to lead to calls for the resignation of key figures involved in the debacle, ranging from Met commissioner Cressida Dick to Labour MP Tom Watson.

The report will also say Watson, Labour’s deputy leader, panicked police into interviewing Brittan when he intervened in the case. He denies the claim and points out a letter he wrote was seen by police only after they had already quizzed the former politician, who was gravely ill.

Operation Midland launched in 2014 after claims from Nick – later revealed to be a liar called Carl Beech who in July this year was jailed for 18 years. Beech had alleged he was a victim of a VIP paedophile ring that had also killed three boys. All his claims were made up.

Concern over the authorities’ failures over decades to tackle child sex abuse led the government to set up the independent inquiry into child sexual abuse (IICSA) with Ben Emmerson QC, a respected human rights barrister, as lead counsel. He has since stepped down from the role.

Emmerson said that he interviewed Beech, forming the view his claims were fabricated. He then met Scotland Yard detectives in September 2015 and warned them their witness was a fantasist. Emmerson said: “[Beech’s claims] had a ring of outlandish fabrication about them. I also received a full briefing from the Operation Midland investigation team … I expressed my view to the investigating officers that Carl Beech’s allegations were inherently implausible, and that there were grounds to suspect him of attempting to pervert the course of justice.”

Henriques’ report will say the Met should have realised much sooner Beech was a liar and should have heeded the warning. Emmerson said he told police to check Beech’s computers to prove or disprove his claims, but they declined: “If such a search had been carried out at the time, it is very likely that Beech’s story would have been exposed as fabrication much earlier than it was.”

Most of Henriques’ criticism is focused on the police, but some is directed at Watson. A previously unseen chapter of Henriques’s report details the police’s handling of a separate claim against Brittan, that he raped a woman in 1967. Police eventually dropped that investigation, which was called Operation Vincente.

Facebook Twitter Pinterest The report says a letter from Labour deputy leader Tom Watson ‘panicked’ the police into interviewing Leon Brittan. Photograph: Sean Smith/The Guardian

Henriques will say a letter from Watson about the case put pressure on detectives and panicked them into interviewing the former Conservative home secretary.

The letter was sent to the then head of the Crown Prosecution Service, Alison Saunders, who has said police had interviewed Brittan, in May 2014, before she forwarded Watson’s letter to the Met. Watson, who was a backbencher at the time, has denied the letter was driven by any political malice.

The Met apologised for not telling Brittan before his death on 21 January 2015 that the claim against him would not be pursued. He said he had never met the woman who accused him.

On Thursday, Watson accused Henriques of inaccuracies and said the former judge had not even put the claims to him ahead of writing his report. Watson told the Guardian: “The report doesn’t make clear the key point that Lord Brittan was interviewed by the police before they received my letter.

“It therefore cannot be argued that it was pressure from me that led to Lord Brittan being interviewed.”

Watson added: “The police asked me to encourage the hundreds of people that came to me with stories of child abuse to report their stories to the police. That is what I did.”

The home secretary, Priti Patel, is expected on Friday to refer matters from Henriques’s findings to Her Majesty’s Inspectorate of Constabulary. The exact scope of referral is yet to be determined but may include the inspectorate overseeing the Met’s progress in adopting recommendations from the Henriques report and from the Independent Office for Police Conduct’s (IOPC) report, which is due early next week.

The version of Henriques’s report released in November 2016 was redacted in part because it recommended action against Beech, who had not yet been tried.

Henriques is understood to be most concerned about his finding that detectives misled a judge to gain search warrants, and the chapters of the report released on Friday will give more detail.

A judge was told Beech was a “consistent” witness but not told of seven undermining factors. The then police watchdog, the Independent Police Complaints Commission, investigated and decided the officers should not face any disciplinary charges, to the dismay of police critics and Henriques.

At a press conference in December 2014, the Met described Beech’s claims as “credible and true”. The force later apologised for this and said the detective who said it went further than he should have.

That same month DS Kenny McDonald, who was in charge of the investigation, said police policy dictated that officers believed a victim unless evidence emerged to undermine their account.

He said experienced detectives from two teams had concluded that Beech’s 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,” he said.

The Met has paid compensation to the families of Bramall and Brittan but has been unable to agree a settlement with Proctor. He says the police targeting of him over the bogus claims cost him his job and home.