The result sees OMD losing an estimated £25 million in billings, after handling British Gas' brand campaigns for the past ten years.
Carat's appointment comes after the decision to axe separate campaigns for British Gas' engineering, telecommunications and core gas businesses in favour of an umbrella brand identity based around the idea of the "British Gas home".
This triggered a centralisation of the planning and buying arrangements, which were split between OMD, handling the core brand campaign, and Carat and All Response Media, which divided the direct response work. The media for British Gas' direct marketing is to be combined with the above-the-line business. This will push the total value of the account toward the £60 million mark.
"Although this has been a very difficult decision, we are all excited by the opportunity to deliver our total communications strategy through one agency, Nick Smith, British Gas' director of marketing and strategy, said.
In a statement announcing the appointment, British Gas said the review had sought a "data-driven, media-neutral strategy. Carat launched a data planning division, focused on the Royal Mail account, three years ago.
Centrica, the company that owns British Gas, had originally envisaged an even wider centralisation, with Zenith's £20 million AA media business and Manning Gottlieb OMD's £500,000 Goldfish account both combined with the British Gas business. The consolidation was dropped in favour of a process which will see each piece of business reviewed in turn.
MindShare, which does not hold any Centrica business, was included on the initial shortlist but dropped from the review at an earlier stage.