The company, owner of McCann Erickson, Lowe Worldwide and Initiative Media, had already revealed the delay in filing its results, and that it had uncovered $145m (£76m) in overstated revenues, relating to acquisitions made between 1996 and 2001.
The company had told investors it would update them on the situation this week, but in a statement issued last night it said it still can not give a date for the problems to be sorted out. It said that sorting out the problems could affect its 2006 fiscal year too.
However, it did go into more detail about the "material weaknesses in internal controls" that have led to the delay. These include problems with inter-company transactions, client contracts and fee arrangements, real estate leases, and not properly communicating policies on financial controls throughout the organisation.
Interpublic's chairman and CEO Michael Roth said: "At this time, we are relying on extensive manual review procedures to augment our existing control processes in order to protect the integrity of our financial statements."
On a brighter note for the group, Interpublic said that net debt stood at $300m, the lowest for the company in three years.
The problems come at a crucial time for Interpublic as it faces the challenge of hanging on to one of its biggest clients. Shareholders are already wary over the news that General Motors is reviewing its $2.8bn media account.
Roth, who only took up his role in January this year, said: "The filing delay is not related to the high quality of our professional offerings, as evidenced by recent significant account wins. Nor is it a reflection of our company's true potential. We will, of course, provide further information on our situation as warranted."
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