Airport operator BAA again faces having to sell Stansted and either Glasgow or Edinburgh after losing a Court of Appeal ruling.
The Competition Commission ruled in March last year that the airports, along with Gatwick, must be sold, but BAA successfully appealed.
But now the finding that there was "apparent bias" in the commission' s ruling has been overturned.
That means the forced sale of the airports is now back on track.
BAA has already sold Gatwick for £1.5bn to US-based investment fund, Global Infrastructure Partners.
Spanish-owned BAA operates Heathrow, Southampton and Aberdeen, as well as Stansted, Glasgow and Edinburgh.
It had successfully appealed against the compulsory sales to the Competition Appeal Tribunal in March - arguing that one of the competition commission inquiry panel had a connection with the owners of Manchester airport, who had shown an interest in buying Gatwick.
But Lord Justice Maurice Kay, Lord Justice Jacob and Lord Justice Patten found that the tribunal was wrong to find "apparent bias" and restored the commission's original decision.
BAA said it was disappointed with the decision and would seek permission to appeal to the Supreme Court.