The Friday afternoon report on the FT's Alphaville blog claimed the deal had been in preparation for six months and would be priced in the region of $15 per share.
IPG’s share price was last week trading at under $10 per share, having dropped from earlier in the month after it revealed a 3.1% drop in second-quarter profits.
On Friday an unknown buyer acquired 40 million Interpublic shares at an estimated cost of $400m.
Over the weekend, Publicis Groupe issued a statement denying "having engaged in any discussions with Interpublic and confirms it has not commissioned any bank to undertake such discussions".
Speculation of a Publicis Groupe-Interpublic deal has done the rounds before, with suggestions that Publicis Group chief executive Maurice Lévy would like to close his long reign with a defining move.
Lévy is due to step down soon and the Publicis Groupe board is set to initiate the process of searching for his successor in September.
Publicis and Interpublic are the world’s third- and fourth-largest holding groups respectively.
The world’s fifth largest, Dentsu of Japan, is set to complete a transformative £3.2bn acquisition of Aegis in the next few months, bringing it much closer to Interpublic’s size.Follow @DanFareyJones