J. Walter Thompson was in the running for the task at an early stage in the pitch but later pulled out of the review.
The company started the pitch process, which was conducted though the AAR, back in January. Greggs Bakeries, owned by Greggs Plc, which also owns the Bakers Oven stores, has more than 1,000 outlets in the UK.
The emphasis of the marketing drive will be to raise the company's profile in the south of England - Greggs is already an established brand both in the north of England and in Scotland.
The company already runs regional TV campaigns in the North, Scotland and the Midlands and it is considering running a national television campaign.
In the company's AGM statement last week, Greggs' chairman, Ian Gregg, stated that "retail expansion is on target, with 17 net openings in the current year giving us a total of 1,161 shops across the UK".
The company is in the process of changing its Birketts outlets to the Greggs fascia, planned for completion by June. In January a Greggs spokesperson said: "We are now looking to build a successful national brand."
No-one at RPM3 or Greggs was available for comment as Campaign went to press.