A police probe into claims former Liberal MP Sir Cyril Smith was released by officers after being found in possession of child pornography has closed due to a lack of evidence.

Simon Danczuk, the current Rochdale MP, made a series of claims about the late Smith's sexual activities, including that he was stopped by police who discovered he had child pornography while travelling on the motorway through Northamptonshire.

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But Northamptonshire Police said they could find no records or witnesses to corroborate the allegations, made by the Labour MP in his book Smile For The Camera: The Double Life Of Cyril Smith.

Detectives interviewed five people who came forward but could only provide third party accounts which could not be used in evidence.

Mr Danczuk, two former chief constables and 60 police staff were also interviewed, the broadcaster said.

Northamptonshire Police said Special Branch "had undertaken a manual trawl of their archives" and the Crown Prosecution Service (CPS) had also searched its archives "for relevant information" but had found nothing.

"To date, no witness has been found who saw Smith in custody or was involved in his arrest and no reports of the alleged incident have been uncovered," a statement from the force said.

"Furthermore, no witnesses have been found from Special Branch or any other senior influence, while a primary witness - a canteen worker mentioned in the book - cannot be identified."

Smith, who died in 2010, was the subject of allegations and investigations over decades during his career, first as a prominent local councillor in Rochdale and later as the town's Liberal MP. He was never prosecuted.

The force added the cold case investigation could be reopened if further information came to light.