The Transport Secretary Patrick McLoughlin will give the government's response to the report into airport expansion in the House of Commons today.
The official position of the coalition is that no new runways will be giving the green light in the course of this Parliament.
Even though ministers cancelled the previous plans by the last Labour government for a third runway at Heathrow, it is thought the Chancellor, George Osborne, has now become convinced of the need to expand Britain's busiest airport.
Many Conservative MPs privately acknowledge the inconsistency of the Prime Minister's arguments: How can the UK be in a "global race," they argue, if business can't connect to the emerging economies of the world because Heathrow is full?
But the pledge not to support a third runway at Heathrow was crucial to winning a number of key marginal constituencies in West London at the last election.
The Tory MP Zac Goldsmith, who represents Richmond Park, is bitterly opposed to expansion and is threatening to resign if Heathrow is allowed to build a new runway.
Officially, the coalition's position will remain unchanged for another 18 months.
The Airports Commission, which released its interim report today, is not due to unveil its final recommendations until the summer of 2015. The astute ones amongst you will know that is, conveniently, after the date of the next General Election.
Reports in the last few weeks suggested the Chancellor had persuaded the Commission to include a non-Heathrow option, so it didn't look like a done deal a year and a half before the final report is due.
Treasury sources insisted to me, however, "there was always" a non-Heathrow option in this interim report.
Even though the government might be softening the ground, no runway at Heathrow will be permitted before 2015.
But it seems clear that ministers have acknowledged that the status quo at the UK's busiest airport is no longer an option.