As well as a third runway, ministers are considering an end to runway alternation. This would mean a major rise in the number of aircraft using the existing two.
Shadow transport spokeswoman Theresa Villiers made the call during a Commons debate. John Stewart, chairman of campaign group HACAN, called it a welcome and very significant move.
Transport secretary Geoff Hoon denied Villiers's claim that the decision has already been made, insisting that approval relies on air quality and noise criteria being met.
The debate came after more than 140 MPs, including 50 Labour members, signed an early-day motion urging consideration of alternatives to a new runway.