Cameron's warning over third runway

Some observers have suggested that the green light given by Transport Secretary Geoff Hoon for expansion at the west London airport represents a guarantee that it will go ahead, as the scheme will be too far advanced by the time of the next election for an incoming Tory administration to halt it.

But Mr Cameron flatly rejected this assumption, stating: "The third runway is just not going to happen."

In an interview with The Guardian, he insisted that he did not believe that a Tory government would be required to pay compensation if it scrapped the scheme.

With the announcement triggering what is expected to be a protracted planning process, little financial outlay will have been made by Heathrow's owners BAA by the election, he suggested.

And he left businesses in no doubt that any investments made on the assumption that the third runway will be built will be put at risk if the Tories win power.

"What business needs to recognise is that the third runway is just not going to happen," said Mr Cameron. "There is such a coalition of forces against it. There's such an environmental case against."

The announcement on Thursday was greeted with horror by environmentalists, local residents and many Labour backbenchers, who have opposed the scheme on the grounds that it will worsen air quality and noise levels around Heathrow and set back Britain's efforts to cut emissions of climate change gases.

A group protesting against the Government's decision to expand Heathrow smashed glass doors at the Department for Transport (DfT), it said.

The group, which calls itself The Climate Suffragettes, said three women wearing red sashes hurled bricks at the doors of the Westminster building at 4am on Friday. The bricks were wrapped in notes that read: "No third runway, the Suffra-jets are back," they said.