Gordon Brown has been pressed to confirm MPs will get a vote on plans for a third runway at Heathrow.
Conservative MP John Randall, whose Uxbridge constituency borders Heathrow, asked him for a pledge that any further expansion would go to a Commons vote.
But he would only say there would be a debate and a possible planning inquiry.
BBC political editor Nick Robinson said it was clear there would not be a vote and Mr Brown was determined the third runway would go ahead.
About 50 Labour MPs, as well as the Conservatives and Liberal Democrats, oppose the expansion plans.
The government has said it backs a third runway in principle, subject to noise and air pollution limits, and access concerns.
At prime minister's questions, Mr Randall reminded the prime minister of his comments last year, when there were protests on the roof of Parliament against Heathrow's expansion.
He said: "You will recall on February 27 last year you said: 'The message should go out today very clearly that decisions in this country should be made in the chamber of this House and not on the roof of this House.'
"Will you now make a pledge that any further expansion at Heathrow will have a vote in this chamber?"
The prime minister replied: "There was a decision in principle, subject to air noise, subject to pollution and subject to access. The secretary of state is examining this matter, he will report to Parliament.
"There will be a debate about what he says in the questions that follow, if the matter was accepted and a proposal was put by the secretary of state it would go to a planning inquiry."
Protests have been growing in anticipation of a decision, which the government has said will be made this month, having already been put back from December.
Campaigners have bought up some land earmarked for the construction of a third runway, should it get the go-ahead and say thousands of people have signed up to buy a plot - in an effort to frustrate the expansion plans.
But BA chief executive Willie Walsh said expanding Heathrow was the only "credible option" and said all the environmental concerns could be met while airport operator BAA argues Heathrow needs a third runway to remain competitive globally.
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