Omnicom's PHD has handled the bmibaby account since the airline launched nine years ago.
The client has given PHD a four-month notice period to enable a smooth transition. The move is understood to be driven by a desire to centralise its media business.
Martin Smith, head of marketing at Bmi Baby, confirmed approximately £4.6 million of media spend will be split equally over offline and online channels.
MPG Media Contacts won the £11m bmi media account, following a review in July 2008. PHD was the incumbent on the main bmi brand at the time.
Bmi's creative business is not affected by the review and will continue to be held by M&C Saatchi.
Bmi flies to destinations in Europe, Middle East, Africa and central Asia from its operational base in Heathrow, while bmibaby operates short-haul flights in the UK and to European cities.