CALLS are mounting for a public probe into how police investigate their own ranks.

The national force has one senior officer - former Central Scotland assistant chief constable John Mauger - effectively on holiday spending months of leave accrued while he was gardening leave on full pay for three years.

And earlier this month Police Scotland saw a five and a half year investigation into two constables - Amanda Daly and Andrew Reid - collapse at Glasgow Sheriff Court with a sheriff ruling they had no case to answer.

MSP Linda Fabiani of the SNP has already asked for Holyrood's justice committee to look at internal investigations amid concerns of a "possible culture of secrecy" over such inquiries.

She is backed by Aamer Anwar, a solicitor representing Ms Daly and Mr Reid, who has lodged a formal complaint that the force's counter-corruption and professional standards units are a "law unto themselves".

Now Labour's Graeme Pearson, a former deputy chief constable, has added his voice to the calls, saying he thinks the Scottish Police Authority (SPA) should look into how rank-and-file are investigated.

The MSP said: "I think the SPA should satisfy themselves that things are as they should be. They seem to be absent from the field on this issue.

"With some cases lasting years there is a danger of the system becoming a joke. Mr Mauger's example is almost a joke already."

Mr Anwar made a series of allegations on the conduct of internal investigations, especially over alleged data protection breaches, in a letter to Chief Constable Sir Stephen House seen by The Herald.

He said: "The length of time taken to investigate Constables Daly and Reid was an abuse in itself."

The solicitor cited claims that a senior officer had anonymously phoned Crimestoppers with bogus allegations against an officer that was subsequently investigated.

Mr Anwar said: "Within Police Scotland there is a perception that there is no accountability or transparency of the Counter Corruption Unit, in that they are responsible for investigating complaints against themselves.

"It is hardly surprising then that ordinary police officers feel that they are a 'law unto themselves'."

Police and prosecutors take data protection breaches very seriously, arguing officers can have access to highly sensitive personal information and intelligence about organised crime.

The Scottish Police Federation, which represents rank and file officers, has previously argued that the test for prosecution should be what its members do with information, not whether they access it.

Chief Superintendent Ellie Mitchell, head of Professional Standards, declined to comment on Mr Anwar's letter because it was being formally considered as a complaint against the police.

She said "Police Scotland takes seriously any allegation of misconduct or criminal behaviour by officers or staff and as an organisation expects the highest professional standards to be adhered to at all times in order that the public can have confidence in us.

"There are well-established processes in place for both internal and external, independent investigation of such allegations."

The SPA said its complaints and conduct committee looking at the scale of misconduct and criminal investigations in to officers.

A spokeswoman said: "Although complaint and conduct investigations for officers under the rank of Assistant Chief Constable are the responsibility of Police Scotland, the SPA does have oversight of all complaints.We continue to actively monitor the situation with Police Scotland."