Theresa May has insisted Heathrow Airport can be expanded without breaching legal limits on air pollution as she sought to head off a damaging revolt from grassroots Tories.
The announcement on Tuesday that the Government has finally opted to build a third runway at Heathrow brought a renewed threat of legal action by councils in the area, including Windsor and Maidenhead, which includes the Prime Minister's constituency.
Local Conservative MPs have also voiced anger over the decision, with Zac Goldsmith dramatically quitting as MP for Richmond Park, forcing a by-election in which he will stand as an independent.
In the Commons, Mrs May was challenged at Prime Minister's Questions by Tania Mathias, the Conservative MP for Twickenham, who said that legal air quality limits were already being breached around the airport.
The Prime Minister was adamant that all three airport expansion schemes considered by the Government - including the third runway at Heathrow - were compliant with air quality standards.
"The Government looked very closely at this issue of air quality and environmental impact. The evidence shows that air quality standards can be met as required by all three of the schemes, including the north-west runway at Heathrow," she told MPs.
Her intervention is unlikely to convince councils in the area to abandon their challenge. Earlier, Lord True, the Conservative leader of Richmond Council, confirmed the authority was taking legal advice.
"The fact that the Government have already delayed action for a year results from us reminding them that they haven't yet fulfilled things which they are required to do under existing law," he told the BBC Radio 4 Today programme.
"Heathrow is operating illegally at the moment. This airport is busting air quality limits. It is already putting out over London 40% of the noise pollution in Europe associated with airports. We will be looking at every aspect of the decision."
With the Tories announcing they would not field a candidate in the by-election to avoid exacerbating divisions in the party, Lord True said he would be campaigning for Mr Goldsmith.
"Zac is an elected MP, he was elected with a huge majority, he has got a mandate against Heathrow," he said.
"He is fulfilling a promise he made - unlike most politicians. Certainly I will be campaigning for him to be returned in a resounding vote against Heathrow from this part of London."
Meanwhile Transport Secretary Chris Grayling confirmed the runway could built over the M25, saying it would be "cheaper and quicker" than tunnelling under the motorway.
"I am, of course, very concerned to make sure that, as this runway is built, it doesn't cause massive disruption on the M25, so I think this is a sensible way," he told the Today programme.
"It is a very gentle hill up which the planes would take off rather than a flat surface. It's what happens at very many airports around the world."
Mr Grayling said that while he was "sorry" Mr Goldsmith had chosen to resign, the Government had to act in the interests of the whole country.
"Of course it is difficult if you're directly affected by the change - and I feel deeply sorry for those who are - but ultimately in politics you have to do what's best for the whole United Kingdom, and if we're going to create a country that works for everyone in it then this is the kind of thing we're going to have to do to make a difference for the future," he told Sky News.
Speaking at digger company Miller UK, in Cramlington, Northumberland, Mr Grayling said "grown-up modern politics" meant the Government would respect the views of Mr Goldsmith and Foreign Secretary Boris Johnson.
Asked how people in the North would take a visit from him to announce another huge infrastructure project for the South, he said the runway was needed to create a country that worked for everyone.
"It will create better connectivity for Newcastle around the world, it means better connectivity for the whole North East region around the world, this is part of the strategy that ensures we have a country that works for everyone," he said.
"The Foreign Secretary has strong views about this and has for many years. The decision taken was unanimous and we are firmly of the view this is the right strategy for the nation, we need to deliver it and we will deliver it.
"In a grown-up world of modern politics sometimes you will find situations where people have a long-standing view that they have articulated in public on many occasions and you have to respect that.
"We respect Zac's decision on a point of principle, he has said for a long time that he feels passionately about this.
"We also think he's a really good constituency MP and so we have taken the decision to respect that experience and that contribution that he makes to his community and not stand against him."