The decision has been led by a change in the airline's commercial focus in light of rising fuel costs and falling revenue.
Euro RSCG, which handles advertising for both airlines, has declined to repitch for the business.
Media is handled by PHD, but only the media account for bmi will go up for pitch, with bmibaby retaining the agency to oversee its media planning and buying.
Bmi has drawn up a shortlist of three London-based creative agencies and three media agencies, including the incumbent, to battle for the £8 million advertising, digital and media accounts.
Bmibaby, the low-cost airline, has shortlisted a number of regional agencies for its £3 million advertising business. The bmi pitch will take place in July with a result expected at the start of August.
A bmi spokeswoman said: "We have a change of commercial focus within the business and need our advertising to work harder for us. We want to retain and grow our share of a smaller marketplace and that requires stronger commercial focus throughout the business, which starts with our advertising."
Mark Cadman, the chief executive of Euro RSCG, said: "We are really proud of the work we have done for bmi."