A watchdog investigating Police Scotland for spying on journalists and their sources visited Glasgow last week as part of its high-level probe.

Members of the Interception of Communications Commissioner’s Office (IOCCO) are believed to have interviewed senior officers over a row that is set to be examined by a Holyrood committee.

It has also emerged that Justice Secretary Michael Matheson has rejected a meeting with the Scottish Newspaper Society (SNS), which represents the industry, on the single force snooping on reporters.

In 2014, it was revealed that police had used the Regulation of Investigatory Powers Act (RIPA) to identify journalists’ sources, a practice which was argued to be in breach of human rights law.

The RIPA allows public authorities to access “communications data” of individuals, which includes phone records and details of emails and texts.

An IOCCO review found that 19 police forces had used the RIPA in this way over a three-year period, involving 608 applications.

The last UK Government then required police forces to seek judicial approval before using the RIPA to determine contact between reporters and their sources.

However, the IOCCO announced last month that two unnamed forces had broken the new rules, one of which the Sunday Herald revealed is Police Scotland

The force and the Scottish Government, which learned of the breach before the IOCCO report was published, have refused to comment.

It is understood an inspection team from the IOCCO, which is based in the Home Office, visited Police Scotland in Glasgow on Wednesday to continue its probe into the violation.

The head of the watchdog is Joanna Cavan, who is described as an “interception, communications data and digital forensics specialist”.

She leads on oversight of intelligence agencies, police forces and other public bodies, and used to manage the Covert Policing Unit at West Mercia police.

Cavan is supported by nine inspectors and two secretarial staff.

The Sunday Mail newspaper last week claimed the breach could relate to its investigation into the former Strathclyde force’s botched probe into the murder of prostitute Emma Caldwell.

The nine-page special report was published on April 5 in the name of the paper’s editor and deputy editor.

John McLellan, the director of the SNS, has requested a meeting with Matheson to discuss a number of issues, including the RIPA, but a meeting with civil servants was offered instead.

“It is disappointing Mr Matheson’s office has rejected a meeting,” he said.

Scottish Liberal Democrat leader Willie Rennie MSP said: “If it comes to light that Police Scotland did spy on journalists without judicial approval, anyone with a liberal bone in their body will be furious at the Scottish Government’s apparent attempts to protect police chiefs. Now is the time for tough questions and transparency.”

A Police Scotland spokesperson said: "IOCCO has clearly set out its rationale for not identifying organisations in its report and therefore it would be inappropriate to comment further."

IOCCO declined to comment.