Shares rose to 113.5p, having leapt 11.42% to 109.5p earlier.
A report on Dow Jones says that analysts see a French or Japanese buyer for Aegis being most likely as US and UK buyers are ruled out of the running.
"It's always easier to see Aegis as a target than it is to find a buyer. Our view is that of the two likely buyers, Omnicom Group is fully served, while Interpublic Group of Companies wouldn't have the wherewithal. WPP Group is fully served in media buying. Havas or Publicis Group more likely."
There are also reports that takeover speculation follows stake-building in Aegis, according to analyst Bear Stearns.
It noted Goldman Sachs Group had passed a notifiable interest level on July 21 and has since acquired a further 4%.
"It is unclear if this is an in-house investment or on behalf of a third party," Bear Stearns said. It also tagged Havas as a potential buyer.
Since Vincent Bollore took over as chairman of Havas in June and restructured the management of the firm, he has said he is committed to building and growing it.
"My plan is to have Havas independent. We are not interested in selling anything," Bollore said last month.
One area more than others Havas needs to grow is media, which it can now only do through acquisition.
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