In recent months, O'Brien, who believes the loss-making Independent newspapers should be sold off, has steadily built up his holding in the company to just over 21%.
Unveiling its 2007 full-year financial results, which showed a 7.7% rise in pre-tax profit to €286m (£224m), INM said it believes O'Brien's "comments and actions" are designed to destabilise the company.
The statement added: "Consequently, the board is now formally declaring Mr O'Brien a 'dissident shareholder', who consistently voices disagreement with the company's strategy while continuing to buy shares, and who is not acting in the best interests of all stakeholders. The company will be responding to, and interacting with, Mr O'Brien on that basis henceforth."
The newspaper group went on to say that since O'Brien disclosed his initial investment in 2006, he has made a "series of personalised, misleading and inaccurate attacks" on the company.
The company's comments were delivered as it revealed a strong set of results for its UK operations, where it publishes the eponymous Independent daily and weekend newspapers. UK revenue increased 18.3% to €268.1m (£210m), driven by strong advertising revenue growth, up 5.5%.
The Independent and The Independent on Sunday titles reported a 2.1% increase in advertising revenues.
Unveiling its 2007 full-year financial results, which showed a 7.7% rise in pre-tax profit to €286m (£224m), INM said it believes O'Brien's "comments and actions" are designed to destabilise the company.
The statement added: "Consequently, the board is now formally declaring Mr O'Brien a 'dissident shareholder', who consistently voices disagreement with the company's strategy while continuing to buy shares, and who is not acting in the best interests of all stakeholders. The company will be responding to, and interacting with, Mr O'Brien on that basis henceforth."
The newspaper group went on to say that since O'Brien disclosed his initial investment in 2006, he has made a "series of personalised, misleading and inaccurate attacks" on the company.
The company's comments were delivered as it revealed a strong set of results for its UK operations, where it publishes the eponymous Independent daily and weekend newspapers. UK revenue increased 18.3% to €268.1m (£210m), driven by strong advertising revenue growth, up 5.5%.
The Independent and The Independent on Sunday titles reported a 2.1% increase in advertising revenues.