Bollore, who owns 29.9% of Aegis and is chairman of its rival marketing services group Havas, is now likely to take six to eight weeks to make a decision about the bid.
The report comes from DealReporter, a news service owned by the Financial Times, which claims two sources told it that Bollore has been working on the bid.
It said the delay is due to a number of factors, including the recent rise in Aegis' share price to as high as 82p last Monday after a report that the company had engaged investment bank Merrill Lynch to advise on the possible sale of Synovate.
Aegis' move has complicated the bid, while the delay is also attributed to weaker than expected results in the company's US operations and the complexity of securing finance for the deal.
The sale of £400m-valued Synovate would require the agreement of Bollore, which banking sources said was likely given his greater interest in the media buying side of Aegis.
German market research group GfK indicated it would be interested if Synovate were available.
A GfK spokesman said: "If Synovate comes up for sale we will take a look. We have authorised capital to spend and a debt structure that would enable us to raise further capital."
Last year GFK lost out on a nil-premium merger with UK market research company TNS after WPP broke in on the deal with its own ultimately successful bid, securing TNS for £1.1bn.
Bollore's progression to readying a bid for Aegis was enabled by the departure of his target's chief executive Robert Lerwill in November.
Lerwill's entrenched opposition to Bollore's interest is not necessarily shared by John Napier, who joined Aegis as chairman in the summer and took the chief executive role on an interim basis on Lerwill's departure.