AN outside probe into an unlawful Police Scotland spying operation linked to the Emma Caldwell murder investigation has become marred by a conflict of interest row.

The Police Service of Northern Ireland is investigating the misconduct aspects of the spying scandal, but it has emerged that PSNI Chief Constable George Hamilton used to work with senior officers who had key roles on the Caldwell case.

The Sunday Herald revealed that Police Scotland’s Counter Corruption Unit (CCU) had used its spying powers in 2015 in a flawed bid to flush out sources in a newspaper investigation.

A tabloid newspaper had exposed Strathclyde Police’s failed investigation into the murder of Caldwell, a sex worker whose body was found in woods near Biggar in 2005.

Within days of the media report being published the CCU used the Regulation of Investigatory Powers Act (RIPA) to find out whether two serving and two retired officers had a role in the story.

However, not only were the four individuals entirely blameless, but the force forgot to seek judicial approval for the snooping operation – a clear breach of the rules.

The row triggered Holyrood hearings and an unprecedented Investigatory Powers Tribunal, at which a remedy for the breaches was discussed.

Police Scotland then got an outside force, Durham Constabulary, to carry out an inquiry and detectives recommended misconduct probes for officers who were involved in the 2015 RIPA breaches.

After Durham handed over its report to Police Scotland, the single force asked the PSNI to undertake the misconduct investigations into officers - who are still serving - over the RIPA row.

However, Police Scotland’s decision is under the spotlight over the PSNI Chief Constable’s previous stint at Strathclyde.

Hamilton joined Strathclyde as Assistant Chief Constable (crime) in 2009 – the year after the case against the original suspects in the Caldwell case collapsed – and stayed in post until 2011.

According to the 2010 Police and Constabulary Almanac, Strathclyde’s deputy head of Crime Division Operations, a post that fell under Hamilton’s remit, was Detective Superintendent John Mitchell.

Mitchell, who retired as an officer but is now working for the Police Investigations and Review Commissioner, had a senior role in the original Caldwell probe.

In 2010, the head of the Major Crime and Terrorist Investigation Unit, which was also part of Hamilton’s empire, was Colin Field, who had also been a key figure in the Caldwell investigation.

In the 2011 Almanac, Field’s successor at MCTIU is listed as John Cuddihy, who had also worked on the probe into the sex worker killing. Field and Cuddihy are now retired.

The documents also reveal Hamilton’s historic link to a senior officer who is now a key figure in the current CCU row.

The officer listed as the 2010 contact for the “force intelligence bureau”, which was also under Hamilton’s umbrella, was Clark Cuzen.

Cuzen later joined the CCU in Police Scotland and worked in the Unit at the time of the 2015 RIPA controversy.

There is no suggestion of any wrongdoing by Hamilton, Mitchell, Field or Cuddihy. Hamilton had no role at all on the Emma Caldwell case.

However, a senior policing source said of the PSNI probe: “This looks like a conflict of interest.”

Scottish Tory MSP Liam Kerr said: “These revelations raise concerns as to whether there is a conflict of interest in the investigation by the PSNI."

A spokesperson for Police Scotland said: “The Police Service of Northern Ireland (PSNI) has agreed to carry out an independent misconduct investigation on behalf of Police Scotland following the conclusion of Durham Constabulary’s enquiry into complaints about Regulation of Investigatory Powers Act (RIPA) procedures and associated matters. Assistant Chief Constable Mark Hamilton from PSNI has been appointed as the Investigating Officer.

“The officers subject of this investigation have been notified and served with conduct related papers in line with the The Police Service of Scotland (Conduct) Regulations 2014 as is usual procedure during such investigations. Police Scotland will fully support the PSNI investigation.”

Mark Hamilton, an ACC at the PSNI, said: “The PSNI agreed to carry out this independent misconduct investigation on behalf of Police Scotland. The team reports directly to Deputy Chief Constable Fitzpatrick and I am satisfied it is an independent investigation.”