Police Scotland has been described as “reckless” by a privacy watchdog that found it breached the law on intercepting communications five times to spy on journalists’ sources.

The Interception of Communications Commissioner’s Office (IOCCO) found that Police Scotland had used powers without judicial approval to intercept phones and emails to discover either a journalist’s source or communications of those suspected to have been acting as intermediaries between a journalist and a suspected source.

The people … must be assured they are free to raise concerns with journalists without the fear of being exposed Alison McInnes, Lib Dem MSP

Sir Stanley Burnton, the interception of communications commissioner, also found that the police had failed to give due consideration to article eight or 10 of the European convention on human rights.

He concluded that four individuals targeted by Police Scotland were adversely affected by these contraventions and that the failures identified were reckless. Burnton has since written to the individuals, who could engage the investigatory powers tribunal.

The controversy erupted in August after it emerged that an elite anti-corruption unit in Police Scotland had allegedly spied on a freelance journalist.

Assistant chief constable Ruaraidh Nicolson insisted there was no evidence that the breaches, which were all directly connected to one investigation into the alleged unauthorised release of sensitive police information in early April 2015, were intentional, and that a detailed action plan was put in place as soon as the failures had been highlighted.

Both he and the Scottish government’s justice secretary, Michael Matheson – who also noted the implementation of a plan to ensure no repeat incidents – emphasised the IOCCO’s clarification that no journalists were targeted by Police Scotland as part of their investigations.

But Alison McInnes, a Scottish Liberal Democrat justice spokeswoman, suggested this was a spurious distinction: “Trying to uncover a journalist’s source or the details of someone acting between a journalist and a source is in my view just as bad as spying on a journalist. The confidentiality of journalists’ sources is something enshrined in law and no police officer should consider they can breach that.

“The people of Scotland must be assured that they are free to raise concerns with journalists without the fear of being exposed.”

She added: “Both Police Scotland and the Scottish government have known for months that the national force was involved and yet said and did nothing. Now that we have confirmation, it is absolutely vital they are held to account so we can ensure freedom of the press and freedom of speech in this country.”