The company has appointed the AAR to handle the process. A shortlist will be drawn up over the next few weeks, with pitches being held in the new year.
Admiral currently creates its advertising in-house, but now wants to develop a relationship with an external agency.
The winning shop will work alongside its in-house unit on future campaigns.
In recent years, the company's campaigns have featured its brand spokesman, "the Admiral", who highlights the problems that consumers face if they use price-comparison websites.
In one spot, he argues that the sites do not offer details of some of Admiral's best deals, such as its offer of discounts for consumers who opt to take out coverage on multiple vehicles.
Admiral has also used celebrities in its TV ads, including the Olympic rower Sir Steve Redgrave.
In May, Admiral Group appointed MPG to handle its media planning and buying account. The agency won the business after a pitch against MediaCom North and Feather Brooksbank, in a process also handled by the AAR. Mindshare, the previous incumbent, did not repitch.
Admiral offers home, tra-vel, car and van insurance.
The company is part of the Admiral Group, which also incorporates Confused.com, elephant.co.uk and Diamond. Those brands are unaffected by the review.