The site, which launched last summer, appointed MBA to work on its advertising in June last year but has called a pitch for the business.
Agencies are expected to pitch for the account next week. MBA has taken the decision not to repitch, effectively resigning the business.
The AOL-owned site is thought to have called the review after changes to the marketing and management team at AOL, which include the December departure of AOL Europe’s vice-president of marketing and communications, Sarah Gavin.
MBA has created high-profile work for The Huffington Post, which included launch activity comprising an outdoor and Twitter campaign asking commuters to "blog all about it". Activity also included press, taxi and online advertising, as well as mock newspaper stands at stations complete with sellers shouting the campaign strapline.
AOL acquired The Huffington Post for $315 million in February 2011. In the first month of its UK launch, it attracted more than one million unique users. The UK version is based on the US model, which employs a team of journalists and sources material from unpaid bloggers.