The account will now be led by College Hill partners Matthew Smallwood and Justine Warren.
The firm is the UK's largest independent online fashion retailer, with 5.2 million unique visitors a month. The retailer, primarily targeting young adults, was established in June 2000 and listed on London's AIM market little over a year later.
Cubitt had held the account for six years and held off College Hill in a seven-way repitch 18 months ago. Cubitt was subsequently shortlisted in the PRWeek Awards 2009 best City campaign category for its work with ASOS.
ASOS is also understood to have recently engaged Brunswick on a CSR-specific brief.
