The Airports Commission will recommend Heathrow as the preferred site for London’s new runway, sources have told the BBC.
But while backing Heathrow, it may also leave the door open for alternative expansion at Gatwick.
The commission is due to make its recommendation at 07:00 on Wednesday.
With no clear decision, the report is likely to open up splits within the Conservative party over which airport to back .
As the commission is an independent body its report does not bind the government.
A new runway at Heathrow would affect the constituencies of at least five Conservative cabinet ministers, including Theresa May, Phillip Hammond, Greg Hands, Justine Greening and Theresa Villiers.
London Mayor Boris Johnson and Richmond Park MP Zac Goldsmith, who is bidding to replace him, are also opposed to the Heathrow site.
Residents near the airport have already launched legal action against the way the commission has considered the pollution and noise impact of a new runway.