Police are investigating claims that the late Liberal MP Cyril Smith was arrested after being caught with child abuse images in his car but released after making a phone call.
The former MP for Rochdale, who died in 2010, was stopped on the M1 by officers, according to a book on Smith co-authored by the Labour MP Simon Danczuk and Matt Baker.
Northamptonshire police confirmed on Thursday that officers are probing the circumstances around the arrest. It is the third police force to investigate a cover-up around Smith.
A prosecution source has confirmed that a file was referred to the director of public prosecutions “over an allegation or allegations relating to indecent obscene publications”.
Smith was released from police custody after making a telephone call to an unidentified third party in London, Danczuk claimed.
Police say they now believe that Smith was a prolific abuser of boys and should have been charged with crimes more than 40 years ago.
Supt Steve Lingley, from Northamptonshire police, confirmed the inquiry 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.
The force’s police and crime commissioner Adam Simmonds said the suspicion was that officers were “brought under pressure from somebody in London to release Smith”.
Earlier this week, it was claimed an undercover investigation gathering evidence of child abuse claims in the 1980s against Smith was ‘scrapped’ shortly after he was arrested on a separate occasion.
A former special branch officer told Newsnight that officers were threatened with prosecution under the Official Secrets Act if they spoke out about the investigation, and offered the possibility of promotion if they kept quiet.
The Metropolitan police is already examining allegations of a cover-up involving Smith.
The Greater London force has confirmed Smith visited Elm Guest House in west London. There was no evidence found to support the assertion that people were conveyed from London’s children’s homes to be abused, as has been claimed.
Greater Manchester police is examining claims that there was a cover-up among police and local politicians to stop Smith’s activities from emerging into the public domain.
