IN&M holds a 29.9 per cent shareholding in the Tribune's parent company, Tribune Newspapers plc.
According to the union, which is seeking a meeting with a receiver appointed to the title, NUJ members at the Tribune are holding a chapel meeting today and expect to hear later if the paper will be published next Sunday.
In a statement yesterday, IN&M said it had committed adequate funding to enable the receiver to "assess the prospects for the business of Tribune Newspapers plc as a going concern, including funds to meet staff costs for the month of February".
"This follows a decision by IN&M – which holds a 29.9 per cent shareholding in Tribune Newspapers plc – that ongoing funding of the Sunday Tribune by IN&M is no longer viable," the statement added.
"IN&M remains hopeful that new investors may emerge to safeguard the future of this quality Sunday title."
In a release from the NUJ, Irish secretary Seamus Dooley said staff at the Dublin-based paper were shocked by the announcement.
"Staff were only told of the development at 3.30pm on Tuesday, and advised that the receiver is assessing whether it will be possible to publish the title this week. There had been no prior warning of the threat to the newspaper."
"The financial situation at the Sunday Tribune has been precarious for some time. Over the past 12 months staff, led by the editor and her team, have made a tremendous effort to revamp the Tribune, which occupies an important place on the media landscape in Ireland.
"Staff have also made significant sacrifices by accepting pay cuts, despite the fact that they were earning less than counterparts in other national newspapers. The Sunday Tribune remains a lively, vibrant and authoritative newspaper and its demise would be a major blow to media diversity in Ireland."
At the time of writing Tribune Newspapers had not responded to a request for comment.
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