A hurriedly issued statement yesterday morning said the pair, who also sit on the Aegis board, are stepping down to pursue other professional interests.
However, insiders suggest that Kemoun and Rebbouh, who have worked with the group for 18 years, have repeatedly clashed with the Aegis plc chief executive, Doug Flynn, over the future direction of the company.
In particular, it is believed that Kemoun and Rebbouh were unhappy with the levels of investment Flynn has allocated to the Aegis research business Synovate at the expense of the core media planning and buying agency network. One insider said: "Their vision isn't the same as Doug's and they hadn't endeared themselves to him."
However, sources in Paris said that Flynn had held talks with WPP over a possible deal leading to a clash with the pair. WPP sources denied this.
Kemoun and Rebbouh were appointed to the Aegis board in September 1992.
Their links with the company go back to 1985 when they founded 2010 Medias in association with Carat France. The architects of Carat's expansion in the 90s, the pair are considered central to key client relationships including Danone, Renault and Carrefour.
They sold 2010 in 1991 and became major shareholders in Aegis. According to Aegis' 2002 annual report, the pair were each paid £524,000 last year and with more than 786,000 shares apiece, were by far the biggest shareholders among the directors. Each also held nearly six million share options at the end of 2002.
Aegis Media was created in June and was designed to increase co-operation between Aegis' media brands Carat, Vizeum and Posterscope.