The Aegis cash-and-share offer for Mitchell has already been "unanimously recommended" to Mitchell shareholders by the board.
The Mitchell Communication Group has been the largest media buyer in Australia for the last five years, and accounts for 10% of all media buying in Australia.
The group subsidiaries include Sydney-based digital printing company Coleman Group, which specialises in the production of outdoor and indoor signage for the sporting and exhibition industry.
Mitchell Group founder and executive chairman Harold Mitchell said: "Aegis is the best placed of the global agency groups for the convergent future, with a strong focus on digital and media. We are convinced they have enormous growth ahead of them and having Mitchell as part of their global network will be an important part of achieving that."
Aegis Group is one of the world's biggest marketing communication companies. It has a presence in more than 82 countries and last year made an underlying profit of £170.3m on revenues of £1.35bn.
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