POLICE Scotland has launched an internal probe after it “lost” a key part of the intelligence case behind an illegal spying operation into journalists’ sources.

The force cannot find CCTV footage seized to establish links between a serving officer and a retired policemen as part of a leak investigation into the unsolved murder of sex worker Emma Caldwell.

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The 27-year-old was found in woods in rural Lanarkshire weeks after she was reported missing from Glasgow in April 2005.

Green MSP John Finnie said: “This ‘loss’ will fuel the belief that sinister elements are at work here trying to frustrate any legitimate enquiry.”

The force has admitted unlawfully using the Regulation of Investigatory Powers Act (RIPA) last year to find out how a newspaper got its information for articles in April last year about the Caldwell case.

The Counter Corruption Unit (CCU) acquired telephone data for two serving and two retired officers they wrongly believed were linked to the leaking of confidential police records.

The four individuals, who did nothing wrong, took their cases this month to the Investigatory Powers Tribunal, at which Police Scotland admitted to the “reckless” breaches.

Police Scotland also confirmed this week that the chief constable of Durham Constabulary had been asked to lead an outside probe into the scandal.

However, a key element of the intelligence case behind the flawed CCU mole hunt has vanished.

A report by Her Majesty’s Inspectorate of Constabulary in Scotland (HMICS), which examined the CCU’s unlawful behaviour, found: “As part of investigating connections between serving and retired police officers, we learned that CCTV disc footage was seized by police officers from the CCU Intelligence Section.

“We enquired into the audit trail of this particular CCTV disc footage and established that the data whilst seized had not been lodged as evidence and was lost.”

The watchdog added that the “handling” of the CCTV footage “had not been properly adhered to by the CCU Intelligence Section and that evidence had been lost”.

It is believed that the CCTV was retrieved after a former officer spotted two of the four targeted individuals chatting.

The HMICS report stated: “I also found that confidential information provided by a retired police officer was not sufficiently recorded, risk-assessed and lacked proper evaluation. This information unduly influenced the CCU intelligence development activity.”

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A policing source said: “It’s unbelievable there was such a casual disregard for evidence handling.”

A second insider said the CCTV loss meant that part of the “flimsy” intelligence case behind the CCU investigation had now disappeared.

Police Scotland’s probe into the missing footage will be conducted by officers unconnected to the breaches, the CCU, or to the re-investigation of the Caldwell case.

Liberal Democrat MSP Liam McArthur said: “This whole episode has been murky from the outset. We need full transparency over how the unlawful spying came about and reports that crucial pieces of evidence have been misplaced are a concern. The investigation into the activities of the counter corruption unit must have the full cooperation of Police Scotland or people will have no confidence that the problems we have seen will be addressed.”

A Police Scotland spokesperson said: “This matter will be fully investigated, in line with the HMICS recommendation, and considered as part of the wider review of counter corruption being taken forward.”

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The spokesperson declined to comment on whether any officer had been served misconduct papers as a result of the loss.