Mr O’Hagan, who was secretary of the NUJ’s Belfast and district branch, was killed in front of his wife Marie by members of the Loyalist Volunteer Force (LVF) on 28 September 2001.

Nobody has been successfully prosecuted in connection with the killing.

On the eve of the 23rd anniversary of the killing, NUJ general secretary Michelle Stanistreet and assistant general secretary Séamus Dooley sent a joint letter to Northern Ireland secretary Hilary Benn, UK prime minister Keir Starmer, Taoiseach Simon Harris and Northern Ireland’s first minister Michelle O’Neill and deputy first minister Emma Little-Pengelly.

Mr Dooley said: “For many years the NUJ has marked the anniversary of the vile murder of Martin O’Hagan with calls for an independent investigation. Polite acknowledgements have not been followed up by action but we continue to demand an external investigation into the murder and the failure to properly investigate it.

“There are so many unanswered questions, so many issues which remain unresolved, despite the fact that the identity of those suspected of ordering and carrying out the assassination of Martin O’Hagan is well known, with names being published in the media with impunity.

“Since last year’s anniversary we have had a new Northern Ireland secretary, a new British prime minister, a new taoiseach along with a new first minister and deputy first minister. In that context, we are seeking support for a new investigation, led by an international figure and independent of those who have so far failed to properly investigate the murder.

“The strong suspicion of collusion with the security forces and the suggestion that the PSNI may have had prior knowledge of the plan to murder Martin O’Hagan must be thoroughly interrogated.

“The murder of Martin O’Hagan was a watershed moment for journalism in the UK and Ireland. The failure to properly investigate it has cast a long shadow and continues to embolden those who seek to silence journalists.”

Amnesty International has backed the NUJ’s calls for a fresh investigation into the killing.

Patrick Corrigan, Amnesty International’s Northern Ireland director, said: “The public execution of Martin O’Hagan was designed to send a clear message to journalists in Northern Ireland that they are not safe.

“It is simply unacceptable that, in the 23 years since Martin O’Hagan was shot dead, not a single person has been held accountable. This failure has created an environment of impunity for those who continue to threaten journalists in Northern Ireland today.

“It is notable that, in 2024, threats of serious violence continue to be directed at journalists from the very same sort of armed groups which killed Martin O’Hagan.

“We support calls from the National Union of Journalists for a fresh investigation into the killing. Press freedom must be resolutely defended.”