Simon Danczuk (pictured) says everyone but the public seemed to know about Cyril Smith's crimes - but no one was able to do anything to stop him

Countless times I’ve heard the sentence now, and it’s usually followed by an exasperated sigh. ‘Yes, we knew all about Cyril.’

Former Lancashire police officers, Greater Manchester police officers and Metropolitan police officers all knew.

Thames Valley detectives knew. Special Branch knew. Even former prime minister Margaret Thatcher had been warned.

Everyone except the public seemed to be aware that the former Liberal MP for Rochdale and ‘national treasure’ Cyril Smith was a child abuser. But no one, apparently, was able to do anything to stop him.

When, in April last year, I first started to speak about the police’s certain knowledge of Smith’s serial offending after publishing my book on his double life as a child abuser, I was dismissed as a conspiracy theorist.

I had researched Smith and his revolting proclivities for more than two years, so I knew what I was talking about. Yet when I mentioned the word ‘cover-up’ it provoked cynical laughter among some of the old guard in Parliament.

Slowly but surely, though, a sober realisation has dawned upon the political classes that we’re looking at a truly monumental cover up.

The revelations aired on Newsnight this week that in 1981 Cyril Smith was released from a police station hours after he had been arrested at a sex party with teenage boys is simply another chapter in this horrifying story.

We now know that Smith walked free from police stations all over the country after orders were made from on high to release him. He was first investigated by police in the Fifties, and was still being investigated by police as late as 1999.

The same pattern of events unfolds in almost every operation. Police are told to hand over all evidence, files are destroyed and officers are warned never to speak about Smith again, with the added threat of being prosecuted under the Official Secrets Act.

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In this way — as I revealed in my book, serialised in the Mail, which brought this gruesome scandal to public attention — Cyril Smith was allowed to carry on abusing with impunity while his public image as some sort of jovial national uncle remained intact.

If there’s a clearer example of a cover-up than the story of Smith then I’ve yet to see it. But he was not the only rotten apple in the Parliamentary barrel.

There have been any number of allegations of appalling abuse by prominent members of the Establishment made by child victims, as well as complaints that they were ignored by the police or hushed up.

It is claimed, for example, that the police deliberately sat on inquiries into Elm Guest House in Barnes, South-West London — an establishment widely described as a gay brothel — allowing dozens of boys to continue to be abused by a VIP paedophile ring.

Slowly but surely a 'sober realisation' has dawned upon the political classes that we’re looking at a truly monumental cover up over Cyril Smith (pictured)

Police are accused, too, of failing to act on sex parties at the now notorious Dolphin Square complex of flats — home to large numbers of VIPs and MPs — following the intervention of ‘prominent people’.

Scotland Yard stands accused of orchestrating no fewer than 14 paedophilia cover-ups, and these cases have now been referred to the Independent Police Complaints Commission.

The IPCC’s announcement this week of its involvement is the strongest indication yet of the extent of the abuse and subsequent cover-ups. And yet I fear these cases may just be the tip of the iceberg.

To understand why, we have to go back to Cyril Smith and ask the question: why was he allowed to get away with it for so long? Why would a Liberal MP with no prospect of ever holding high office be given so much protection? Why was he given a pass to do as he pleased and prey on innocent boys all over the country?

It’s not as if MPs could not be prosecuted for this type of offence. In 1962, for example, Sir Ian Horobin, Tory MP for Oldham, the neighbouring seat to Smith’s Rochdale constituency, was jailed for four years after being found guilty of indecent assaults on boys.

But by the time police moved to secure a prosecution against Smith later that same decade, something had changed. The Director of Public Prosecutions was prepared to listen to representations from politicians, and stamped ‘Not in the public interest’ on Smith’s case files.

I now believe that this is because a paedophile network - perhaps involving prominent politicians and policemen - had been established, and, if charged, Cyril could have blown it wide open.

The gravity of the allegations, and the cover-up charges against Scotland Yard (pictured), 'cannot be overestimated'

His protection from prosecution continued into the 1970s as paedophiles in Parliament became organised and pressure was put on police to discontinue investigations into child abuse where MPs and VIPs were involved.

You might, like those MPs who thought of me as a conspiracy theorist, dismiss all this as simply unbelievable. But once you’ve felt, as I have, the sense of anger of police officers who were investigating these types of offences at the time, it doesn’t seem fantastical at all.

Time after time, I’ve witnessed cold, sustained fury from police officers explaining how investigations into serious criminal behaviour ground to a halt once they found out MPs were involved. ‘It was disgusting,’ one told me. ‘We had huge rows about it. Officers were not happy.’

I have also spoken to victims who allege MPs abused them at a young age. And we’re not talking sexual peccadilloes here: we’re talking violent, sadistic rape. One man told me how his legs had been broken as a young boy. ‘They took great pleasure in causing us to scream in pain,’ he said. It was sickening stuff.

The gravity of the allegations, and the cover-up charges against Scotland Yard, cannot be overestimated. This is why it’s so important that the IPCC investigation — as well as the statutory inquiry set up by Theresa May into historic child abuse — is allowed to get to the truth of what happened during the 1970s and 1980s when abuse in and around Westminster was rife.

As long as Westminster provides a safe harbour for paedophiles, then we’ll continue to let the most vulnerable people down

I know some MPs are worried about how this could damage previously unblemished political reputations.

But such concerns cannot be allowed to override the vital priority of delivering justice to countless boys who were treated as playthings by a vicious, unaccountable elite.

Parliament now has to focus on making sure Britain is able to protect children from abusers much more effectively. For, as long as Westminster provides a safe harbour for paedophiles, then we’ll continue to let the most vulnerable people down.

Uncovering what could well be one of the great scandals of our age is not going to be easy, as there are plenty of people determined to keep the lid on this dreadful stink.

Police officers — such as the ones I have interviewed — are the key, and that’s why the Prime Minister needs to provide them with an unambiguous, cast-iron guarantee that they can speak out on what they know about child abuse investigations featuring VIPs.

The Official Secrets Act, so often used to block evidence concerning Cyril Smith, should be no bar to the truth being told.

Nor should we be under any illusion that these terrible crimes belong to a bygone age. They’re still with us today. I know only too well from the scandal in my own constituency of Rochdale, with its Asian grooming gangs, that police are still struggling to deal with sex abuse wherever it crops up.

But it’s not just places like Rochdale and Rotherham. It’s time to shine a spotlight on the capital, where I’m convinced even greater failures have occurred.

‘It suits us fine that the focus is on provincial places like Rochdale and Rotherham,’ a serving Met police officer told me last week. ‘What’s going on there doesn’t even begin to tell the story in London. It’s a lot worse here because there’s big money involved. The Met has ignored organised child abuse for years.’

From Islington to Lambeth to Westminster, we’ve seen successive child abuse scandals that have often generated more questions than answers in terms of where culpability lies.

These scandals are unlikely to go away until politicians reform our battered child protection services and help oversee a different approach to the crime from police and other protection agencies.