The detective who first investigated a rape claim against Lord Brittan has revealed he feared the probe would turn into a "baseless witch-hunt" more than a year before the peer died without knowing he had been cleared.
Paul Settle also claimed he had been removed from his role leading the controversial VIP paedophile inquiry after Labour deputy leader Tom Watson intervened in the case.
In blistering testimony to the Commons Home Affairs Committee that raised serious questions for Scotland Yard, the Detective Chief Inspector suggested the force acted illegally by later interviewing Lord Brittan under caution while he was seriously ill.
He was also highly critical of Mr Watson, accusing him of "undermining" officers and "betraying" him by complaining about the probe in a letter to the Director of Public Prosecutions.
Asked if he was surprised the investigation was reopened after Mr Watson's letter, the detective told MPs he said he was "extremely disappointed" and "rather shocked", adding: "I saw it as a betrayal. I saw it as a very low blow to be perfectly honest."
The MP later apologised for the distress caused to Lord Brittan's widow and expressed "sadness" over the officer's remarks - but defended his role in the case.
The former Conservative cabinet minister died in January without being told he would not face action over a claim, made to police in November 2012, that he raped a 19-year-old woman known as "Jane" in 1967.
Police determined that the case should not be pursued in September 2013 but the investigation was reopened and Lord Brittan was quizzed in May last year while suffering from terminal cancer.
Mr Watson faced intense scrutiny after it emerged that the interview was carried out shortly after he wrote to the DPP Alison Saunders asking for the case to be reviewed.
Mr Settle, who initially led the investigation, told the committee he decided any action would be "grossly disproportionate" and would have no legal basis, adding that the case fell "at the first hurdle".
Reading aloud from his log notes which documented why he took the decision, he said: "There is no right of anonymity for persons arrested for sexual offences and furthermore there is considerable media intrusion regarding arrestees.
"At the moment Lord Brittan is of interest to other aspects of a parallel investigation, and to arrest or interview him now would, I feel, jeopardise any potential inquiries as this would be nothing more than a baseless witch-hunt."
He described how Mr Watson's letter "confused matters considerably", and said: "It shook confidence within the team because it has kind of undermined us."
It also emerged that he had been sidelined after raising his concerns about Mr Watson's interventions.