IAG has made a binding agreement to buy BMI for £172.5m in cash.
"Buying BMI’s mainline business gives IAG a unique opportunity to grow at Heathrow, one of our key hub airports," said Willie Walsh, chief executive of IAG.
"Using the slot portfolio more efficiently provides the option to launch new longhaul routes to key trading nations while supporting our broad domestic and shorthaul network."
Walsh confirmed that there would be job losses at BMI to bring the airline back to profitability.
Virgin Atlantic has requested UK anti-trust authorities to halt the deal, which it says would “tilt the competitive landscape dangerously” toward BA as IAG will now own 53% of the slots at Heathrow. The deal is still subject to clearance from European competition authorities.