The move is likely to trigger a bidding war for Aegis, which owns the Carat and Vizeum media networks, as well as hundreds of research and digital agencies.
Early on Wednesday, Aegis confirmed it had received a preliminary approach of 140p per share but refused to say from which company. "The approach is preliminary in nature and there can be no certainty that an offer will be made," Aegis said in a statement.
The Publicis approach follows last month's news that Vincent Bollore, the chairman of Havas, had built up a 6 per cent stake in Aegis. Havas and other networks are likely to consider bidding for Aegis. Analysts predict that Publicis may have to pay more before an offer is accepted.
Aegis shares rose from 120.5p at the start of trading on Wednesday to 143.5p by mid-afternoon.
Aegis sources said Omnicom had made an equivalent 140p offer last year, which was rejected by the Aegis board.
Since then, Aegis has acquired several media, digital and research companies under its chief executive Robert Lerwill.
An Aegis spokesman refused to comment on who was behind the bid, while Publicis did not return calls.