The IPT is examining alleged unlawful covert surveillance of journalists in a case brought by Barry McCaffrey and Trevor Birney, who produced the groundbreaking documentary No Stone Unturned about the 1994 Loughinisland massacre.

The tribunal has already heard that the PSNI launched a covert surveillance operation following their arrest in a bid to unmask one of their sources.

The NUJ’s application to be joined in the case was granted by the IPT despite opposition from counsel on behalf of the Metropolitan Police.

The union was represented by Brenda Campbell KC and Sean Mullan BL, instructed by Olivia O’Kane, a partner in DWF’s Belfast office.

NUJ assistant general secretary Seámus Dooley said: “Even when the people under surveillance are not journalists, if they are in regular contact with journalists and are being monitored, that is equally disturbing.

“The NUJ fully supports the actions of the journalists and have been represented at the previous hearings, as have the International Federation of Journalists, Amnesty International and the Committee for the Administration of Justice.

“The decision to lodge this application is a response to the serious revelations which have caused such grave concern to our members.

“We also welcome the appointment of Angus McCullough KC to conduct an independent review of the force’s alleged use of surveillance against journalists, lawyers, police regulators and NGOs. That is a process separate from this IPT hearing and is a very welcome development.”

The case resumes on 1 October and is expected to last at least three more days.

The IPT previously accepted an application by NUJ member Vincent Kearney and the BBC to be joined in the case. The tribunal president confirmed that a separate hearing will be held into the matters they have raised.