13/06/2012 13:12 BST | Updated 13/06/2012 13:42 BST

David Cameron Refuses To Rule Out Third Runway At Heathrow U-Turn

David Cameron has refused to rule out a U-turn on blocking a third runway at Heathrow airport, in the strongest indication yet that the government is rethinking the decision.

Responding to a question by Tory MP Zac Goldsmith at Prime Minister's Questions on Wednesday, Cameron said: "I can tell him the coalition position has not changed, but clearly we must not be blind to considerations about airport capacity."

The comment was immediately siezed upon as a sign that the government is looking again at its decision to block plans for a new runway, which would ease Heathrow's overcrowding problem but would see many homes being demolished and a large increase in noise and pollutants in the area west of London.

Goldsmith, an environmentalist, is fiercely opposed to a third runway at Heathrow, as is Mayor of London Boris Johnson, who has indicated he would resign if the government announced the go-ahead for such a major expansion.

The previous Labour government was in favour of a new runway at Heathrow but the Tories went into the last election with a manifesto pledge to ensure it was never built.

If the government were to U-turn on its policy towards Heathrow it would be its most significant and high-profile policy reversal.

The coalition has already u-turned around thirty times since coming to power, something which is causing concern among Tory backbenchers who are trying to defend policies to voters.

It would be opposed by the Liberal Democrats but it is unclear what Labour's position would be. Labour recently acknowledged that a new runway would not be built.

Heathrow is running at near 100% capacity and although the government is trialling the use of "mixed mode" takeoffs and landings on both of its runways, business leaders worry the airport will lose its global hub status within a decade as larger airports in France, Germany and the Netherlands become more attractive to major airlines.

Boris Johnson is keen for an entirely new airport to be built east of London, either on the north Kent coast or on a reclaimed island in the Thames estuary.

Other options include expanding Gatwick Airport and creating a high-speed rail link to Heathrow.

But speculation that the government might be on the verge of a U-turn on the third runway has been mounting for three months. Earlier this week ministers said that the airport operator BAA would be allowed to submit its proposals for a new runway, as part of a consultation on "sustainable aviation policy" this month.

Any U-turn by the government would likely come in the form of a manifesto commitment at the next general election and would be unlikely to involve any direct action before that.

For the Tories any U-turn would carry significant electoral hazards - Zac Goldsmith says he would not stand as a Tory MP if ministers buckle under the pressure to change policy, and there are more than a dozen marginal seats west of London and the surrounding area where Heathrow expansion would be a significant electoral issue.

One of the leading opponents to a third runway at the last election was Justine Greening, the MP for Putney in southwest London and now transport secretary.