Labour MP Tom Watson put pressure on police investigating false allegations of sex abuse by Westminster VIPs, a report said.
A review found officers probing an allegation of rape against the late former home secretary Lord Brittan were “fearful of media criticism and public cynicism”.
Former High Court Judge Sir Richard Henriques said the case should have been dropped at the very latest after the Tory politician was interviewed under caution.
But he said officers may have been “in a state of panic” over a letter sent by the Labour deputy leader on House of Commons notepaper.
Scotland Yard said the force was not aware of the letter at the time a review of the case was conducted, while Mr Watson said Sir Richard’s report “contains multiple inaccuracies” regarding his involvement.
The force published the findings into Operation Vincente – the investigation into an allegation that Lord Brittan had raped a 19-year-old woman in 1967 – for the first time on Friday.
Sir Richard wrote: “I have concluded that the errors in this investigation were largely attributable to the fact that Lord Brittan was a prominent person and there was a desire amongst senior officers to reassure the public that if they come forward the police will investigate a complaint thoroughly no matter whom the allegation is against.
“I have concluded also that investigating officers were fearful of media criticism and public cynicism and sought protection from it by the Crown Prosecution Service (CPS).”
The report said officers tried to persuade the CPS to give pre-charge advice three times, and on each occasion the case should have been dropped.
“The officers responsible were under a positive duty to make a decision themselves and failed to do so,” Sir Richard wrote.
Lord Brittan died, aged 75, before he was publicly cleared of the allegation. He was also a suspect in Operation Midland, which centred around false claims made by fantasist Carl Beech, then known as “Nick”.
Sir Richard’s report said: “There can be no doubt that Tom Watson believed ‘Nick’ and it should be stated that he had previously provided the MPS with information leading to convictions in other cases.
“His interest, however, in both Operation Midland and Operation Vincente created further pressure upon MPS (Metropolitan Police Service) officers.”
Mr Watson said: “The report doesn’t make clear the key point that Lord Brittan was interviewed by the police before they received my letter.
“Former Deputy Assistant Commissioner (Steve) Rodhouse has been clear that the letter did not influence the investigation, and ex-Director of Public Prosecutions Alison Saunders publicly confirmed that my letter was not received by Police until after the interview.
“It therefore cannot be argued that it was pressure from me that led to Lord Brittan being interviewed.”
Mr Rodhouse, who is now a senior director at the National Crime Agency, was referred to the police watchdog over the inquiry, but the body did not take it on as an investigation.
He said: “My decision-making on Operation Vincente was driven by the need to ensure that the public could have confidence that the MPS had thoroughly investigated the allegation made against Lord Brittan.
“This was important in light of the many allegations that the police had historically overlooked, made against prominent public figures.”