Oneworld alliance members British Airways, American Airlines and Iberia face having to release Heathrow slot pairs under obligations arising from their co-operation on transatlantic routes.
Before the European summer 2014 season, the carriers must release slots at Heathrow - or Gatwick, if requested - for one daily service to Boston; one daily to Miami, with the option of one stop; and - if competitor airlines' frequencies on the route fall below 13 per day - three services per day to New York.
The requirements are conditions of European and US antitrust clearance for a transatlantic joint venture that begun in 2010.
Meanwhile, IAG - the parent group of British Airways and Iberia - must make a further three slot pairs available for competitors to operate from London to Aberdeen, Cairo, Edinburgh, Moscow, Nice or Riyadh as a condition of permission for the acquisition of BMI.
Interested parties have until 22 August to apply for the slots.
The transfers are being handled by slot coordinator ACL and the antitrust monitoring trustee CompetitionRx.