Denis O'Brien has raised his stake in Independent News & Media to 8.35 per cent from 7.28 per cent.
"As at close of business on 4th May 2007, I had an interest (directly or indirectly) in 64,165,000 ordinary shares ... which represents approximately 8.35 per cent of the total issued share capital," Mr O'Brien said in a notification to the company.
Mr O'Brien has been steadily increasing his holding in a media group that is already 26 per cent owned by its Chief Executive Sir Anthony O'Reilly.
The publisher of newspapers in Ireland, Britain, South Africa and Australia has bought over 10 million of its own shares in recent weeks, prompting media speculation the company may be defending itself against a possible bid by Mr O'Brien.
INM is also set to ask shareholders at a meeting later this month to back proposals to tighten the company's rules on the disclosure of stake sizes.
Analysts have pointed out that Mr O'Reilly himself has little room for manoeuvre, as his stake, when combined with shares owned by his sons and other board members, is already close to the 30 per cent threshold that triggers an automatic, full takeover bid.