Transport Secretary Justine Greening claimed there was "cross-party consensus" that a third runway at Heathrow was not the right thing for Britain.
The Conservative MP who fought against expansion at the site at the general election said the coalition agreement was "very clear" with no support for an extra runway.
She admitted it "would be difficult for me" to sit in a cabinet that supported a third runway, but said that was not going to happen, despite calls for the government to accept the call from some business leaders for airport expansion.
Ms Greening said the "facts remains as they were at the time of the election" over a new runway, and the prime minister was "very clear" over Heathrow expansion.
She was later asked about the West Coast mainline rail contract, and said there was no reason to delay signing the 15-year franchise for FirstGroup to take over from Virgin Trains in December.
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