British Airways won "a year's free run" at Heathrow when European regulators fast-tracked approval for the takeover of BMI, according to Virgin Atlantic Airways.
Virgin Atlantic chief commercial officer Julie Southern hit out at the time it would take to decide on the use of Heathrow slots surrendered by BA, and chairman Richard Branson said: "I thought there would be an outcry when the EU waved this through.
"They messed up the competitive landscape. It seems a travesty."
Southern told Travel Weekly: "It is astonishing they can approve a merger of this magnitude in 35 days and it takes a year to decide on the slots.
"That is a year's free run for BA when the competition has come out of the market."
Southern insisted Virgin wants to operate the full complement of 12 pairs of slots surrended at Heathrow and said the carrier would argue against splitting these as the regulator has suggested.
She said: "The worst decision would be to give a few slots to submission A, a few to B and a few to C. Then no one ends up providing effective competition to a dominant carrier that has become even more dominant.
"We have been an effective competitor to BA for 28 years and we hope to do as good a job on short haul as we have to date on long haul.
"But having acquired BMI, BA will essentially have a whole year without any competition."
BA parent International Airlines Group took over BMI in April following approval of the purchase from Lufthansa in March.
The deal was agreed in late December and BA began integrating BMI Mainline this month, with the process due for completion by the start of the winter schedule.
VIrgin Atlantic has said it will appeal against the regulator's decision, but that is likely to take at least two years. Southern said: "We hope the remedy will be reviewed so maybe more slots are divested."
The process of allocating the 12 pairs of slots surrendered by BA is likely to take until the end of the year, meaning the slots will not be in use until the start of the summer 2013 schedule.
Southern said: "We'll make a submission about the 12 slots available in late summer and expect some indication of the thinking around the start of the winter season, with a decision late in 2012 ready for a summer 2013 start.
"We expect to make a compelling case, but the submission goes into a process that has no transparency."
Southern was speaking on Virgin Atlantic's inaugural service to Vancouver yesterday. Branson was in Vancouver to greet the arrival.