May warned she faces legal challenge if MPs back Heathrow expansion

The Conservative leader of Windsor and Maidenhead Council, which covers Mrs May’s own constituency, said they were ready to join other local authorities in mounting a legal challenge.

The warning came as Transport Secretary Chris Grayling announced that, after years of wrangling, the Cabinet had finally given the green light for the expansion of Heathrow to go ahead.

After the Airports National Policy Statement (NPS) was formally laid before Parliament, the Department for Transport confirmed there would have to be a Commons vote by July 11.

Despite the strong opposition of a number of Tory MPs with constituencies around the airport – most notably Foreign Secretary Boris Johnson who has previous threatened to “lie down in front of bulldozers” – ministers appeared confident of winning.

Although the Labour leadership reacted cautiously to the announcement, the plan – which is backed by some of the big unions – was welcomed by many of the party’s MPs, while it also has the support of the SNP.

A more significant hurdle could come from local councils who are threatening to take the case to the courts unless concerns of residents around the airport were properly addressed.

Simon Dudley, the leader of Windsor and Maidenhead Council, said they were among four authorities considering legal action along with the environmental group Greenpeace.

“Let’s be clear here, if it doesn’t satisfactorily address concerns, then if MPs vote in favour of adopting this National Policy Statement, that opens up a six week window to a legal challenge and there will be a legal challenge,” he told BBC Radio Berkshire.

In his Commons statement, Mr Grayling was keen to stress that Heathrow expansion would bring benefits across the country with a boost of up to £74 billion to passengers and to the wider economy.

“The time for action is now. Heathrow is already full and the evidence shows the remaining London airports won’t be far behind,” he said.

“Despite being the busiest two-runway airport in the world, Heathrow’s capacity constraints means that it is falling behind its global competitors, impacting the UK’s economy and global trading opportunities.”

Mr Grayling said local communities would receive a £2.6 billion package towards the costs of compensation, noise insulation and improvements to public amenities that was among the “most generous in the world”.

He expected to see for the first time a six-and-a-half hour ban on scheduled night flights, and said the scheme would only go ahead if ministers were satisfied it would not impact on the UK’s clean air obligations.

Mr Grayling would not be drawn on the whipping arrangements for the Commons vote, although in an apparent reference to Mr Johnson’s opposition, there had been “almost entirely universal support” for the plan at Cabinet.

Earlier however Treasury Minister John Glen indicated there would be “understandings” for MPs like Mr Johnson with constituency interests when it came to the vote.

“Obviously, constituency commitments would normally come first,” he told the BBC Radio 4 Today programme.

Former transport secretary Justine Greening, whose Putney constituency lies under the Heathrow flight path and who also opposes the plan, said she did not expect the vote to be whipped.

“I think the Prime Minister is very clear, this is not a party political issue. These are local MPs who need to represent our local communities,” she told the Today programme.

Th announcement comes after the independent Airports Commission concluded in 2015 that an new north-west runway for Heathrow was the best option for delivering extra airport capacity – a decision endorsed by ministers in October 2016.