WPP owns 22% of Tempus and has proposed to swap its stake for a 5% stake in Aegis, which is valued at £1.3bn compared with minnow Tempus's £332m.
As consolidation continues in the advertising sector, neither group is expected to remain independent for much longer.
Sir Martin Sorrell, chief executive of WPP Group, is understood to have been rebuffed at least once already by Aegis chief executive Doug Flynn, but talks are reported to be back on.
Tempus chief executive Chris Ingram is said not to be in favour of such a deal and is looking to find a white knight investor. One such partner could be its larger rival Aegis. The two media-buying companies are known to have held talks.
Other possible partners include Lord Bell's Chime Communications, while another could be Cordiant.
Cordiant is in the process of doing a deal with Publicis over their jointly owned Zenith Media Group. Publicis wants to merge Zenith with its Optimedia arm to form a new group in which it would own 75%, leaving Cordiant with a minority stake.
Tempus owns the CIA Group media planning and buying network and Aegis owns Carat.