The head of the organisation investigating claims of gross misconduct against Scotland's top police officer has told MSPs the probe would not be prejudiced if he returned to work.

Police Investigations and Review Commissioner Kate Frame told MSPs she informed police oversight body the Scottish Police Authority (SPA) of Pirc's view last month.

In a letter to a Holyrood committee convener, she wrote: "I advised the new Chief Officer of the SPA on 11 December 2017 that, as things currently stand, there would be no prejudice to the Pirc investigations if the Chief Constable was not suspended."

Her intervention comes as controversy surrounds the decision not to allow Chief Constable Phil Gormley to return to active duty, with claims that Justice Secretary Michael Matheson intervened in the matter.

Mr Gormley has been on a leave of absence since September as Pirc investigates allegations of gross misconduct, which he denies.

The SPA board gave Mr Gormley its unanimous backing to return to work following a meeting on November 7, asking him to confirm whether or not he would resume full duties on November 10, documents obtained by the Sunday Herald revealed.

The newspaper published extracts of a letter from Mr Gormley's lawyer David Morgan to the SPA's then chairman Andrew Flanagan, saying his client was "concerned that his return was delayed following intervention" by Mr Matheson.

Now Ms Frame has written to Holyrood's Public Audit and Post-Legislative Scrutiny Committee to "ensure that there is no ambiguity about the circumstances surrounding my engagement (or rather the lack of it) in the process surrounding the Chief Constable's absence from duty".

She said the SPA did not consult her before Mr Gormley's September announcement he would go on a leave of absence or in the time between him doing so and Mr Flanagan leaving his post.

Nor did the SPA consult her in advance of Mr Flanagan taking the proposal for Mr Gormley to return to work to the Justice Secretary, she said.

The first time her input was sought was December 4, when new SPA chief officer Kenneth Hogg asked her about the potential impact of Mr Gormley returning to work before investigations concluded.

Mr Frame told MSPs: "Had my views been sought at the outset of these investigations, I confirm that I had real and significant concerns that the Pirc investigations may have been prejudiced, if the Chief Constable had not been suspended.

"My concerns mainly arose from the fact that a large number of the witnesses were police staff from the federated ranks and civilian staff who worked within the Executive offices at Police Scotland's Headquarters, Tulliallan, and therefore in the immediate vicinity of the Chief Constable's office.

"Due to the position of power and influence attaching to the Chief Constable's post, there was a significant concern that those witnesses would not feel free to speak up if the Chief Constable remained in post.

"The Chief Constable's period of leave in England has enabled my investigation to complete interviews of the more junior members of staff, who perhaps had the greatest fear of repercussions and provided them with a safe space to be interviewed without any immediate fears."

She said she was not advised of the SPA meeting on November 7, or the outcome, but if she had been consulted in advance she would have said "most, if not all" of the junior staff had been interviewed and those remaining to be interviewed are senior staff where the "threat of repercussions and damage to career and future promotion prospects is much less".

An SPA spokeswoman said: "The board has agreed that continuing the Chief Constable's period of leave of absence remains an appropriate measure to address investigative and welfare issues for all parties involved."

The SPA is reviewing the decision every four weeks, with the next review planned on January 25.