Police are investigating claims that police found a stash of child sex abuse videos in the car of the late Cyril Smith MP but took no action.

He was arrested on the M1 in Northamptonshire in the 1980s but released minutes later after making a phone call to someone in London, according to claims made in a book about the former Rochdale MP who police now believe was a prolific abuser of children.

Northamptonshire police have now become the third police force to investigate claims of a cover up involving Mr Smith.

The claim is made by the current MP for Rochdale, Labour’s Simon Danczuk, in a book published last year.

In it he alleges that police took the images of child abuse found in Mr Smith’s car into the police station but the material has since disappeared.

Allegations and investigations into child abuse dogged Mr Smith’s career but no charges were ever brought.

Lead officer Supt Steve Lingley, from Northamptonshire police, confirmed the latest cover-up inquiry into cover-up claims on Thursday.

“If Cyril Smith was stopped on the M1, and was arrested by Northamptonshire police, I want to know who those officers are. We want to understand why they released him.

“We want to know if they came under any pressure, who that person was that gave the instruction to be released,” he said.

Northamptonshire’s police and crime commissioner Adam Simmonds said the inquiry would centre on suspicions that police officers were “brought under pressure from somebody in London to release Smith”.

“What worries me the most about that is, if we had reason to hold onto him, this man was released back into the community and continued to perpetrate heinous crimes,” he said.

Earlier this week a retired detective claimed Mr Smith was not taken to court when paedophilia allegations emerged in the 1960s because of concerns that other high-profile sex offenders would be exposed.