Airport operator BAA has been told by the Competition Commission that it should sell Stansted and either Glasgow or Edinburgh airports.
BAA has mounted a string of legal challenges to try to keep them ever since the commission initially ruled in 2009 that the operator must sell three of its seven UK airports.
BAA has already sold Gatwick Airport.
The commission said its decision was "fully justified" and it would publish its final verdict in May or June this year.
It said the initial ruling still held despite the government's decision to rule out further runways at any of London's airports.
"We remain convinced that the original decision to require BAA to divest three airports is the right one for passengers and airlines," said Peter Freeman, the commission's chairman.
"We found that, if anything, since the report there now appears to be greater capacity available, which will increase the potential for competition at the London airports."
He added that Stansted should be sold first as it served more passengers than either Glasgow or Edinburgh airports.
Spanish-owned BAA operates Heathrow, Southampton and Aberdeen, as well as Stansted, Glasgow and Edinburgh.
BAA said it would "carefully consider" the commission's provisional decision before deciding how to proceed and restated its belief that there had been a "material change in circumstances" since the initial 2009 ruling.
In February, the Supreme Court ruled against BAA in its latest bid to stop the sell-offs.
BAA was asking the court – the highest in the UK – to give it leave to appeal against a previous Court of Appeal decision that it should be forced to sell the two airports.