Earnshaw will stay in the post for up to nine months until a replacement is hired. The ISBA Council has created a special three-man committee to come up with a shortlist of candidates.
The committee members are Chris Searle, external affairs director at Bacardi-Martini; Raoul Pinnell, chairman of Shell Brands International; and Phil Smith, commercial and operations director at Camelot Group.
The advertisers' body is currently dealing with issues including the introduction of stricter regulation of food advertising and advertising to children.
Before joining ISBA in November 2000, succeeding John Hooper as director-general, Earnshaw was the global advertising affairs director of Pedigree Masterfoods.
Earnshaw said: "I have thoroughly enjoyed - and continue to enjoy - my role at ISBA but, having completed the agreed six-year term of office and reached the age of 60, it is time to move on."
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