In a report running to more than 200 pages, Angus McCullough KC concluded that there is “no evidence of systemic or widespread use of covert measures” being used by the PSNI against journalists and lawyers.

However, he said police had acted unlawfully in a number of instances, which has drawn criticism from Amnesty International and the Committee on the Administration of Justice (CAJ).

The two groups have jointly written to the Northern Ireland secretary, Hilary Benn, to call for a public inquiry into covert surveillance by MI5 and the establishment of a covert commissioner for law enforcement in Northern Ireland.

Patrick Corrigan, Northern Ireland director of Amnesty International UK, said: “This report exposes a disturbing pattern of unlawful covert surveillance of journalists, with the PSNI showing clear disregard for press freedom and the rule of law.

“The scale of the wrongdoing is alarming, from repeated attempts to identify reporters’ sources to covert operations concealed from oversight bodies.

“But questions remain. How far has MI5 gone in unlawfully monitoring journalists in Northern Ireland? A free press simply cannot function under the shadow of state surveillance.

“That is why we are asking the Secretary of State to set up a full public inquiry. Only full disclosure can restore trust and safeguard the rights of journalists in Northern Ireland.”

Daniel Holder, director of CAJ, said: “The McCullough review has done a thorough job, yet it should not have taken McCullough to get to the bottom of PSNI surveillance.

“There is a current oversight mechanism — the UK Investigatory Powers Commissioners Office — that missed this.

“This was not the mechanism that was supposed to be in place on the back of the Good Friday Agreement.

“The Patten commission had recommended a specific commissioner for covert law enforcement in Northern Ireland to ensure covert policing techniques were being used within the law. This unimplemented commitment should now be taken forward.”

The PSNI’s chief constable, Jon Boutcher, said today: “I am aware of commentary calling for a public inquiry into these matters but this report shows that no such inquiry is necessary.

“As part of the review, Mr McCullough has provided information intended to assist with more accurate media reporting and to help with ‘myth-busting’.

“In his report, Mr McCullough has identified a combination of factors which have had a compounding or ‘snowballing’ effect that has given rise to concerns about the PSNI. His review helpfully corrects this misrepresentation of the position.

“I am conscious that with such a detailed report there will be a natural tendency for some readers to focus only on where we got things wrong.

“I have been reassured by Mr McCullough’s comments about the ‘far more numerous examples of careful and considered practice’ processed by conscientious professionals ‘in full accordance with the applicable legal framework and its safeguards’.

“I hope the Policing Board and the public will be reassured, notwithstanding that we have to tighten up our processes.”