Lerwill's sudden exit caused speculation that the company could soften its position against Vincent Bollore, the chairman of Havas, who holds a 29.9 per cent stake in Aegis. Aegis' shareholders have stopped five attempts by Bollore to win two seats on the company's board, arguing that Bollore's 32.9 per cent stake in the rival advertising group Havas is a conflict of interest.
Last week, The Sunday Telegraph reported that Bollore is more confident of securing board seats.
Aegis has handed Lerwill's responsibilities to the non-executive chairman, John Napier, and has no immediate plans to find a new chief executive.
Lerwill was appointed the non-executive director of Aegis in 2000. He has held the group chief executive role since 2005.