The delay means Interpublic has yet to declare full-year results for 2004 and any figures for the first half of 2005.
IPG confirmed that its second-quarter figures would be delayed and said it might not meet the 30 September target, warning that "extensive work remains to be done".
In a filing to the Securities and Exchange Commission, IPG said the delay was due to the "breadth of the additional internal work and analysis required due to the material weaknesses management has identified as part of its ass-essment of the company's internal control over financial reporting".
IPG also said it was considering whether to reissue financial statements for previous periods. Analysts said that they were not surprised that IPG had delayed the announcement given that it had yet to file 2004 full-year results.
The SEC has been investigating IPG over its accountancy procedures for the past three years and speculation is mounting that it is about to take action.