A watchdog is investigating allegations of criminal behaviour by Police Scotland officers during a rape probe into a sergeant at the single force.

Blair Pettigrew faced trial earlier this year, but the high-profile case was dropped after one of his accusers said she may have mistaken him for "someone else".

The Police Investigations and Review Commissioner (PIRC) has now been instructed by the Crown Office after Pettigrew complained about the force allegedly putting pressure on two women and his doctor.

The Scottish Police Federation, which is representing Pettigrew, said: “We note that matters have been referred to the PIRC and we await the outcome of the investigation.”

Sergeant Pettigrew, 35, from Ardrossan, was accused of raping two females – one while wearing his uniform – and indecently assaulting a third. He was also charged with indecently communicating with a fourth – while using a police radio – as well as allegedly accessing information on women from the Scottish Intelligence Database.

More than 200 witnesses were involved in Operation Mapua and the allegations, which Pettigrew denied, dated from between 2005 and 2015.

As Pettigrew was about to stand trial, the prosecution fell apart after it emerged that one of the women had emailed Police Scotland days earlier to withdraw her allegations.

The email, which was read to the court, stated: “I’ve got a meeting with the [prosecutor] on Friday and I am sorry but I will be telling her I only said what I thought you wanted to hear.

“Your officers wouldn’t leave me alone at my mum’s and then you started coming to my new house. I told you I didn’t know him and you pushed. I have a new life and you made me push into my past. I think I mixed the guy you are after with someone else.”

A Crown Office spokesperson confirmed at the time that prosecutors were not pursuing the case: "It is the duty of the Crown to keep cases under review and after full and careful consideration of the facts and circumstances, including the admissible evidence currently available, Crown Counsel instructed there should be no further proceedings at this time.”

A source close to the case told this newspaper that Pettigrew then made a detailed complaint to assistant chief constable Alan Speirs about the actions of the force during the investigation.

It is believed the letter had twenty-three separate elements relating to on-duty complaints as well as concerns relating to policy, procedure and service delivery.

The insider said one of the complaints centred on claims that two females believed they had been pressurised into making allegations against Pettigrew. It is alleged that one of the women found a police officer so intimidating that she raised concerns with her lawyer about harassment. The woman is said to have refused to sign her witness statement because it was not her words and that it was full of allegations she never made.

In particular, the statement is believe to include claims about the sort of car driven by Pettigrew, but it is alleged the woman denies providing any such details as she said she had no knowledge of the vehicle.

Pettigrew also alleged that police visited his GP and requested his medical records after claiming they were representing his defence team. It is understood Pettigrew claims the doctor was concerned about the approach and immediately contacted him. Pettigrew is said to have made it clear in the letter that no such instructions had been given nor agreed to.

A policing source said the allegations of witness coercion and inappropriate contact with the GP formed the basis of the Crown Office’s decision to make a referral to the PIRC.

In March, a newspaper quoted a friend of Pettigrew saying that the toll on the sergeant had been huge.

“I could see the stress affected his physical and mental health. He put a brave face on but he was a shadow of his former self. There were some dark days when I know for a fact he felt like ending it all.

“He felt totally abandoned by the job he loved and left out to dry. He couldn’t get his head round how he could be charged with raping two women he had never met.”

The friend was also quoted saying: “When he found out the case had been dropped, everyone kept saying ‘You’ll be celebrating tonight’ – but he didn’t. He didn’t feel like celebrating. It should have never happened.

“He has kept his counsel and remained dignified. He and his family have been through it all.”

However Pettigrew, who was initially on restricted duties, faces a potential internal misconduct inquiry and remains suspended on full pay.

A Crown Office spokesperson said: “We can confirm that the Crown Office and Procurator Fiscal Service (COPFS) has instructed the Police Investigations & Review Commissioner (PIRC) to undertake an investigation into allegations of criminality against officers serving with Police Scotland. A report on the Commissioner’s findings will be submitted in due course.”

A Police Scotland spokesperson: "In line with criminal complaints made against police, the allegations were referred to the Crown Office and Procurator Fiscal Service for consideration. The Police Investigations and Review Commissioner has been directed to investigate these allegations. As there are ongoing investigations, we are unable to comment further."

A spokesperson for the PIRC’s Investigation Team said: “We are investigating allegations of potential criminality against officers with Police Scotland and it would be inappropriate to comment further on a live enquiry.

“The matter was referred to the Police Investigations & Review Commissioner by the Crown Office and Procurator Fiscal Service and a report on the Commissioner’s findings will be submitted to them in due course.”