Tom Watson committed a "betrayal" by sending a letter to the director of public prosecutions asking for allegations against Lord Brittan to be reviewed, DCI Paul Settle has said.
Watson was heavily criticised when it emerged that he had written to Alison Saunders in April 2014 complaining about the way a rape allegation against Brittan was being handled.
The deputy Labour leader sent the letter despite having "not expressed any concerns whatsoever" when DCI Settle informed him that Brittan would not be interviewed.
The detective said he and the politician shared a "very strong relationship" - meeting "three or four times" - right "up until that letter arriving".
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.
DCI Paul Settle claims he was moved off an investigation into historical sex abuse "for standing his ground" over the Lord Brittan scandal.
DCI Settle claimed he was undermined and sideline after raising concerns over interventions by Labour's deputy leader Tom Watson.
He was previously the senior investigating officer on Operation Fairbank, the inquiry into historical child sex abuse claims involving prominent figures.
He alleged that he was told "to have nothing to do with the investigation - the whole Operation Fairbank".
Asked what he is doing now, he replied: "Not a great deal."
Paul Settle says he was moved off the investigation for standing his ground. What's he doing now? "Not a great deal"
Tom Watson's information on child sex abuse cases led to three successful prosecutions, DCI Settle said.
Lord Brittan should have been told "as soon as possible" that he would not be charged with rape, Detective Chief Inspector Paul Settle has told MPs.
Brittan died in January without ever being told he would not face action over an allegation that he raped a 19-year-old woman known as Jane in 1967.
A senior police detective has said interviewing or arresting Lord Brittan on suspicion of rape would have been a "baseless witch hunt".
Giving evidence to the Commons Home Affairs Committee, DCI Paul Settle, the head of the Met's anti-paedophile unit, said the case against the Brittan "fell at the first hurdle" in terms of evidence.
He said action would have been "grossly disproportionate".
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.
MPs are set to question Labour's Deputy Leader Tom Watson as well as senior police officers and prosecutors over abuse allegations against Lord Brittan.
A senior police officer decided Lord Brittan did not have a case to answer in late 2013, but the investigation was reopened last year and the former home secretary was interviewed.
Brittan died in January - without being told that the inquiry had been dropped.
Watson has been accused of pressurising authorities into pursuing the peer after writing to the director of public prosecutions.
ITV News UK Editor Rohit Kachroo is listening to the Commons Home Affairs Select Committee.
MPs about to question Tom Watson and senior police officers about abuse allegations against Leon Brittan
Det Ch Insp Paul Settle, head of the Met's anti-paedophile unit, will give evidence first
West Bromwich MP Tom Watson will face a grilling from MPs over his pursuit of sex abuse allegations against the late Lord Brittan.Read the full story ›
Protesters in Nottingham who are campaigning against changes to junior doctors' contracts have told ITV News that the changes will mean tired doctors, that 'could become dangerous'.
They spoke during a rally in Old Market Square where hundreds attended to try to make their voices heard.
Hundreds of people turned out in Nottingham's Old Market Square in protest against changes to junior doctor's contracts.
Junior doctors say the change would see them forced to work longer hours for less pay under extra pressure, and that it would put patients lives at risk.
The Health Secretary Jeremy Hunt has accused the British Medical Association of 'misrepresenting' the Government's plans, but the BMA denies the claims, saying the protests would be a 'wake up call' for MPs.
Jeremy Hunt says the Government wants to stop hospitals being forced to roster three times less medical cover at weekends.
Dr Johann Malawana,chairman of the British Medical Council's junior doctors' committee, has denied the Health Secretary's accusation it has misled doctors over proposed contract changes.
Ahead of a protest rally, he said it would be "a wake-up call for ministers" that pay and working hours reforms are "unacceptable".
In recent weeks the health secretary has acknowledged junior doctors play a vital role in the NHS, which is at odds with his relentless and extremely damaging rhetoric attacking doctors, which has led to the anger on display today.
We have always stated that without the continued threats of imposition and pre-conditions, the BMA would be happy to enter meaningful negotiations. But until the government gives junior doctors the reasonable assurances they are demanding we will continue with our course of action.