By Carole Walker
BBC political correspondent
St Pancras would be linked by rail to Birmingham, Manchester and Leeds
Theresa Villiers has admitted that a few years ago it would have been inconceivable for the Conservative Party to put the brakes on airport expansion, rejecting the pleas of business.
The Conservative transport spokeswoman told me it was a "seriously green decision".
She said the party had agonised over it, but concluded "you can't carry on in the same old way with potentially catastrophic effects on climate change. We've recognised that, Gordon Brown hasn't".
A decision on the third runway is due by the end of this year, though it may slip after Ruth Kelly's decision to stand down as transport secretary.
The signals from Downing Street suggest Gordon Brown wants the expansion to go ahead.
The Tories say the plan to build a new high-speed rail line linking London, Birmingham, Manchester and Leeds would cut Heathrow flights by 66,500 per year.
But the CBI argues that connections to these cities account for a tiny fraction of flights through Heathrow and that a third runway is needed to give businesses the international links they need in the globalised economy.
Ms Kelly said "Even if every flight from Manchester and Leeds/Bradford to Heathrow were replaced by a new high speed line then Heathrow would still be operating at 97% capacity."
The Liberal Democrats have given the plan a cautious welcome but point out that it appears to be at odds with Boris Johnson's plan to build a new airport in the Thames Estuary.
Their spokesman Norman Baker said "You cannot be against London airport expansion and for it at the same time. This shows the hypocrisy at the heart of David Cameron's Conservatives."
Environmental campaigners are also pleased, but fear it could turn out to be an empty promise.
If Labour does give the green light to Heathrow's third runway, it could be too late for the Conservatives to stop it, even if they do win the next election.