The complaint follows reports this morning that the WPP CEO Sir Martin Sorrell is close to deciding to move ahead with a bid for Tempus, which owns the CIA media-planning and buying network.
The complaint to the takeover panel was made by investment bank JP Morgan, which is advising French company Havas on the deal.
A spokesman for Havas said, "The story in the Financial Times creates market uncertainty."
Havas bid 541p in cash and shares, valuing Tempus at £425m -- a 50% premium on the shares prior to the bid. Following the bid, launched on July 19, Tempus shares have been edging ever upwards and at 12.30pm were trading at 588p, up 2.73% on the day.
While Sir Martin and his team at WPP have been poring over the books, WPP shares have steadily fallen. This morning, they were trading down at 691p, down 1.64% on last night's close.
WPP is already the largest single shareholder in Tempus with a 22% stake.
The Takeover Panel oversees the City Panel on Takeovers and Mergers, which is designed principally to ensure fair and equal treatment of all shareholders in relation to takeovers.
The Takeover Panel declined to comment on the matter.