Apax has been researching a possible Synovate deal for weeks, while GfK is believed to have held informal discussions with advisers, according to a report in the Daily Telegraph.
Last year GfK lost out on a nil-premium merger with UK market research company TNS after WPP Group broke in on the deal with its own ultimately successful bid, securing TNS for £1.1bn.
The sale of £400m-valued Synovate would require the agreement of Vincent Bollore, who owns 29.9% of Aegis and is chairman of its rival marketing services group Havas.
Banking sources said his agreement was likely given his greater interest in the media buying side of Aegis.
He had previously worked on a takeover plan of Aegis that would have seen it merge with Havas and WPP buy Synovate.
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Bollore has reportedly delayed his move after it was reported earlier this month that Aegis is considering selling Synovate.