At least six clients, as well as media owners, are believed to have received irregular payments from WPP Italy companies.
Weekend reports claim that WPP Italy's chairman, Bossi, is among those under investigation by Kroll, the corporate investigators hired by WPP chief executive Sir Martin Sorrell.
Bossi is also managing director of an investment bank, Banca Ifis, which is believed to have dealt with WPP Italy media buying agency Mediaclub. Investigators believe that Ifis may have helped Mediaclub to boost operating profits by doing a legal "factoring" deal allowing Mediaclub to reduce some of its debt.
Benatti's financial interest in Mediaclub was behind the friction between Sorrell and Benatti. The two sides disagreed over how much Benatti should be paid as part of the earn-out agreement made when WPP acquired Mediaclub, which Benatti introduced to the group.
The earn-out was calculated on a multiple of operating profits, which the Ifis deal may have boosted.
WPP claims it was not aware of the extent of Benatti's control over Mediaclub until the investigation began.
Benatti has issued a statement saying that his "behaviour has always been completely correct".
There have also been two break-ins at WPP Italy's head office in Milan, in connection with which WPP is now investigating its security firm.
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