It may be "worthy" for the Scottish Government to consider strengthening the police complaints watchdog's powers over the questioning of less co-operative witnesses, Scotland's Police Investigations and Review Commissioner has told MSPs.

Kate Frame, who leads the body tasked with investigating the most serious incidents involving the police, said that of the 91 investigations it has conducted so far, one has been "challenging".

The Police Investigations and Review Commissioner (Pirc) is "constantly reviewing" its powers in light of its experience, Ms Frame told Holyrood's Justice Sub-Committee on Policing.

Pirc is currently examining the circumstances of the death in police custody of Sheku Bayoh last year and Police Scotland's failure to respond to reports of a crash on the M9 which killed John Yuill and his partner Lamara Bell.

Conservative MSP Margaret Mitchell said Ms Frame's predecessor Professor John McNeill had put on record at the committee that "no-one had sought to prevent me from carrying out my functions".

"Is that still the case?" Ms Mitchell asked.

Ms Frame said: "We have been challenged, in particular, in one investigation that we have undertaken.

"The investigation which I speak about - and obviously I am required to discuss this in general terms - has caused us to reflect and we are always constantly reviewing the terms of the legislation and the powers that are available to us in light of that full experience.

"I think that it may be worthy that the Scottish Government potentially look at whether or not there would be an option to consider a precognition on oath of witnesses where they were not being as co-operative perhaps as we would like."

MSPs also highlighted criticism of Pirc by the Scottish Police Federation (SPF), the body which represents rank-and-file officers.

The SPF has complained its members are ''fast losing confidence in the effectiveness and genuine independence'' of Pirc and said officers ''may be told they are witnesses but they are treated like suspects'' in its investigations.

It also said members had reported examples of "being 'interviewed' for hours on end without rest and one apparent witness reported they were only able to use a toilet during a seven-hour interview/interrogation provided they were accompanied by a Pirc investigator".

SNP MSP Kevin Stewart asked: "Do you think that that kind of scenario is the right way to conduct business?"

Ms Frame said she had requested specific information from the SPF to support the perception laid out in its submission to the committee and had not yet received it.

The commissioner added that, on occasions, interviews could take a considerable length of time, but Pirc investigators were well aware of the need for breaks, and it was not practice to accompany witnesses during these breaks.

Ms Frame added: "I can assure that committee that in relation to witnesses that are interviewed by the Pirc, they are treated in a professional manner, with dignity and are treated fairly."

Independent MSP John Finnie said: "You'll understand though there may be concern in some quarters ... let me just be direct, people will be very concerned that you are asking for additional powers, in relation to precognitions."

Ms Frame said: "I think I suggested that the committee may wish to reflect on that and the Government would be the body responsible for providing those powers.

"But there is no weight of evidence yet that would suggest that we have been hindered in any of our investigations."