A third runway at Heathrow is to be approved next month as Gordon Brown prepares to overrule objections made by ministers within his Cabinet.

Allies of the Prime Minister said yesterday that he was as determined as ever to support a plan that he believes is vital for Britain's commerce.

But the possibility of ministerial resignations over the issue is not being ruled out, after a strongly worded intervention by Hilary Benn, the Environment Secretary, about the implications for noise and air pollution. The Government “cannot contemplate” the expansion of Heathrow if it fails to meet European Union pollution targets, he said.

Mr Benn is one of a number of ministers and MPs who are worried about Heathrow expansion on environmental grounds, and others, particularly those in the London area, who fear a backlash from their constituents.

Mr Brown has powerful backers in Lord Mandelson, the Business Secretary, and Geoff Hoon, who became Transport Secretary in the October reshuffle. He has postponed the decision from this month to next, raising speculation that the Government might be having a rethink.

He said that it was to give him more time to consider the 70,000 responses to a lengthy consultation exercise. But sources close to Mr Hoon suggested that he remained in favour of a third runway because it would promote growth and prosperity.

Other ministerial sources said that doubting ministers needed to understand that constructing the third runway was already government policy and that the only question was how they would meet environmental concerns.

One insider said: “This is policy. They can raise their concerns and they will be looked at. But at the end of the day this is policy.”

Sources close to Mr Brown said there was no chance of a U-turn and that he wanted to show that he was taking long-term decisions for the good of the country, even if they were unpopular.

Ed Miliband, the Energy and Climate Change Secretary, is believed to have raised concerns about the impact of expansion on the emission of greenhouse gases without opposing the project as a whole. He was one of those who are believed to have called for a delay on a final decision.

Critics say that expansion of Heathrow would increase noise pollution for households on flight paths, as well as pushing up emissions of nitrogen dioxide and particulates.

Britain has obtained opt-outs from EU air pollution directives, but they run out over the years to 2015, by which time the UK has promised to get emissions down below the limits. Mr Benn told The Sunday Times: “We have to achieve the environmental objectives. We have to honour that commitment and I am determined that we will . . . We have a problem with nitrogen dioxide around Heathrow, principally because of the traffic. Some of it is the aircraft.”

The Environment Secretary played down arguments that “green” technological improvements to air transport would allow the UK to meet EU targets over time, while still expanding airport capacity. “Obviously there is technological change affecting airports, but come what may we will come to a point in 2011, with particulates, and 2015 with nitrogen dioxide, when we will have to be meeting the terms of the directives. That is regardless of whether the decision has been taken. That is a given.”

There would be serious consequences if Britain failed to meet the EU targets, he said. “You are then in trouble with the [European] Commission, you get infraction proceedings and then off you go - which is not something we can contemplate,” Mr Benn said. He made clear that he was taking seriously concerns expressed about pollution in a government consultation about the Heathrow expansion plan.

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