Brearley joined the Yorkshire-based agency almost three years ago and spearheaded its creative approach for the past 12 months.
During his time with Attik, Brearley worked on branding assignments from corporate identity and brand strategy development through to literature, advertising, broadcast and new media production.
Attik, which also has offices in London, New York, San Francisco and Sydney, is searching for a replacement for the 27-year-old design director.
Brearley said he was considering a number of options for his future. He is keen to work more closely with clients when discussing creative concepts.
"In large agencies the creator of a concept often becomes detached from the client. This can lead to the misinterpretation of the critical design subtleties of a new idea or the client's response to that idea," he said.
Brearley's clients at Attik included Nike, AOL, Carlsberg, JobcentrePlus and FPD Savills.
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