Newly-released data for London Heathrow show BMI's slot allocation for summer 2012 will fall by 42% compared with last year.
British Airways parent International Airlines Group is acquiring BMI from Lufthansa, and the data is the first indication of the scheduling impact.
The air transport movement information from slot oversight authority Airport Co-ordination lists BMI as having 461 weekly slots for this summer compared with 800 previously.
It cuts its overall slot share at Heathrow to 4.8% from last year's level of 8.4%.
While British Airways' weekly slot allocation will increase by 154, to 4,194, Lufthansa will gain 248 slots - sharply boosting its total to 776.
British Airways will have a 44% share of the slots but Lufthansa becomes the second-largest carrier at Heathrow with an 8.1% share.