A senior detective has revealed he decided that taking any action against Lord Brittan over a rape allegation would be "grossly disproportionate" more than a year before the peer died without knowing he had been cleared.

Detective Chief Inspector Paul Settle, of the Metropolitan Police, told the Commons Home Affairs Committee he decided the inquiry should not proceed in September 2013.

Lord Brittan, the former Conservative cabinet minister, died in January without being told he would not face action over a claim that he raped a 19-year-old woman known as "Jane" in 1967.

The investigation had been reopened last year and he was interviewed by police while seriously ill.

Mr Settle worked on the case after the complaint was made in 2012.

He told the committee: "On the 4th September 2013, following advice I received from the Crown Prosecution Service...regarding police applying what is known as the code test I decided that the investigation should not proceed any further.

"My reason for that was that I concluded that any action against Lord Brittan would be grossly disproportionate and would not have a legal basis as in order to interview him we would have to have had reasonable grounds to suspect that an (offence) had been committed.

"The investigation had shown that whilst an allegation had been made, the offence had not been made out in law and as such those reasonable grounds had ceased."

Mr Settle revealed he decided not to interview Lord Brittan about the rape allegation because he feared it would be a "baseless witch hunt".

And he said that the late-peer's eventual police interview was illegal because he felt it was not within the Police and Criminal Evidence Act (PACE).

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."

Questioned by MPs, he added: "I think the interview was without grounds and I think that the interview consequently was not within the confines of PACE and unlawful."

He said he did not think there were reasonable, objective grounds for the suspicion of rape based on known facts.

Earlier this month Labour deputy leader Tom Watson came under intense scrutiny after a letter emerged in which he wrote to the director of public prosecutions Alison Saunders in April 2014 complaining at the way the case was being handled.

Mr Settle said he informed Mr Watson of his decision not to interview Lord Brittan and said the MP "didn't express any concerns whatsoever" at the time.

Asked if he was surprised the investigation was reopened after Mr Watson's letter, he said: "If I'm being frank I'm extremely disappointed because I would have thought before doing that I had the type of relationship with him where he could have phoned me to seek clarity, and had I not assured him of that clarity then he was perfectly within his rights to write that letter.

"But I was rather shocked. I saw it as a betrayal to be perfectly honest because I thought I'd been frank and honest with him and transparent from the outset."

He added: "I saw it as a very low blow to be perfectly honest."

Mr Settle, who described the rape case against Lord Brittan as falling "at the first hurdle", said Mr Watson's letter "confused matters considerably".

He added: "It shook confidence within the team because it has kind of undermined us."

The detective said the events took place amid a feeling that "we let people down historically", adding: "We were there to bring perpetrators to justice at the earliest opportunity."

He was asked whether, given the context of revelations of historic abuse, Mr Watson's concerns may have been "above board".

Mr Settle said: "It could be. However... I would have expected him to contact me and just clarify the issue before he made representations. We have kind of gone from a one on the scale to a 10 on the scale."

It also emerged that he had been sidelined after raising his concerns about Mr Watson's interventions.

Asked how he had allowed himself to be undermined, he said: "By doing my job." He said he had felt unable to do his job.

"When I made representations regarding my position on the matter I was told to have nothing more to do with it."

He said he took "no joy whatsoever" in being vindicated and apologised for the distress to Lord Brittan's family, saying: "If in retrospect there's anything which would have prevented that I would have done it."

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 - the whole Operation Fairbank".

Asked by committee chairman Labour MP Keith Vaz if he felt it was because he stood his ground, he replied: "Yes sir."

Asked what he was doing now, he replied: "Not a lot."

Referring to Fairbank, he said police had received in the region of 400 "bits of allegations and information", with only three convictions secured.

The officer added: "An awful lot of information came from the internet and was nothing more than rumour and innuendo."

Mr Settle agreed the way the case had developed and the adverse publicity it has generated could have a more deep-seated impact on the way police investigate similar allegations in the future.

He said: "We are stuck between a rock and a hard place. We are there to sift through things but we are also working within a climate where we have finite resources.

"When those resources need to be directed towards something that is more life threatening or immediate, unfortunately I can see a position where things aren't necessarily looked at because of this.