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 the controversial VIP paedophile inquiry and criticised Labour deputy leader Tom Watson for intervening 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 highly critical of Mr Watson, accusing him of "undermining" officers and "betrayal" by raising the probe in a letter to the Director of Public Prosecutions (DPP).
"I saw it as a very low blow to be perfectly honest," Mr Settle said.
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 matter.
Lord Brittan 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 but the investigation was reopened and the former Tory cabinet minister 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 DPP Alison Saunders asking for the case to be reviewed.
Mr Settle, who initially led the investigation, disclosed that he decided in September 2013 it should not be pursued as any action would be "grossly disproportionate" and have no legal basis.
The case fell "at the first hurdle", he told MPs.
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."
Mr Settle was previously the senior investigating officer on Operation Fairbank, the umbrella inquiry into historical child sex abuse claims involving prominent figures.
He said he was told "to have nothing to do with the investigation".
Asked by committee chairman Keith Vaz if he felt it was because he stood his ground, he replied: "Yes sir."
Asked what he was doing now, he responded: "Not a great deal."
He also apologised for the distress to Lord Brittan's family. "If in retrospect there's anything which would have prevented that I would have done it," he said.
Appearing before the committee, Mr Watson said he was "sad" Mr Settle saw his letter as a betrayal, but he felt there was a risk some alleged victims were not having their voices heard.
"That wasn't my intention, to affect his career, I just wanted to make sure that Jane's voice was amplified in the system," he said.
He also apologised to Lady Brittan, telling the committee: "I'm very sorry for the distress caused. I'm very sorry for the whole wider family. I know they are very angry and they clearly loved Leon Brittan very much.
"They are angry on behalf of their family member and I am sorry."
Reacting to the apology, Lord Brittan's brother, Sir Samuel Brittan, told ITV News: "It's a half way apology, I suppose."
Mr Watson denied he was trying to "micromanage" the police investigation and also rejected suggestions his involvement was politically motivated.
At one point Mr Vaz asked his fellow Labour MP: "You are not Sherlock Holmes, are you?"
After he first raised abuse claims in a dramatic statement in the Commons in 2012, he said there had been an "explosion" in information passed to his office.
It emerged last week that Lord Brittan died before his family were told he would not face action after police repeatedly asked for the CPS to review the evidence.
But Deputy Assistant Commissioner Steve Rodhouse, who took over the investigation in July last year, denied this was because of a "politics of fear" that left celebrities and public figures vulnerable to repeated investigation.
He said: "We wanted to check our decision making."
He also insisted there was a legal basis to conduct the interview with Lord Brittan.
Ms Saunders said the CPS felt that, on the basis of the evidence shown to them, there was not a realistic prospect of conviction.
She said: "We made it clear from the very beginning where we thought the line was in this particular case and we maintained that line. They kept asking us to review that."
Scotland Yard apologised to Lady Brittan earlier this month.