Lerwill's responsibilities will be taken over on an interim basis by the chairman, John Napier.
Aegis has not started a search for a new chief executive, indicating Napier will be at the helm for a while.
Napier took over as chairman from Colin Sharman in July. He also chairman of Royal Sun Alliance and has been chief executive of recruitment company Hays, cash and carry group Booker and water company Kelda.
Napier said: "The board wishes Robert well and thanks him for his contribution to the growth of the company in his period as a non-executive director and chief executive."
Lerwill said: "I have enjoyed my time at Aegis. I leave the group in good hands, with a strong management team capable of meeting the demands of the future."
Lerwill's contract entitles him to a 12-month notice period.
It is understood his departure is related to ensuring the company has a firm direction as the downturn bites, rather than to the company's financial performance.
Napier said Aegis divisions Aegis Media and Synovate will produce operating results "broadly in line with the average of current market expectations".
He added that "the ongoing priorities involve working closely with both companies to ensure that we are in the best possible shape to meet the challenging conditions ahead and ensuring that we also continue to focus on providing a range of services that bring value and insight to our clients, as they too face more demanding and competitive market circumstances".
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