THERESA May has defended her approach to expanding airport capacity after it emerged a final decision by Parliament would not be taken until the winter of 2017/18.

The Prime Minister said a judgment would be made this month on whether to go ahead with a third runway at Heathrow, as the airport insisted there had been "no delay" over the plan.

She said she would settle an issue that politicians had been wrangling over for decades, but the "formal process" would mean a further consultation before a vote by MPs.

Mrs May told the Commons at Prime Minister's Questions: "This is a subject that has been debated, discussed, speculated on for 40 years. This Government will take a decision.

"But there is then a formal process that has to be undertaken so that the Government will identify its preferred option of site. That will then go to a statutory consultation.

"The Government will then consider the results of the statutory consultation and bring forward an airports national planning statement on which this House will vote."

The Davies Commission recommended a third runway at Heathrow, but Mrs May stressed that the other two options in the report - expanding Gatwick or lengthening an existing runway at Heathrow - were "credible and deliverable".

Ms May also moved to head off possible Cabinet resignations by giving ministers freedom to speak out against the Government's decision, which has been fiercely opposed by Foreign Secretary Boris Johnson and Education Secretary Justine Greening.

But she was challenged over the prospect of a new runway at Heathrow by another prominent Tory critic, former cabinet minister Theresa Villiers, who said Gatwick expansion would take less time and money and have a "fraction" of the environmental impact.

"No decision has been taken on the site of airport expansion in the South East," Mrs May insisted.

"The Davies Commission said that airport capacity in the South East should be expanded, the Government accepted that argument.

"The Davies Commission identified three sites, all of which they said would be credible and deliverable, and Government will take a decision this month."

The Government's preferred option will be announced next week but will be followed by a "full and fair" public consultation before a final decision is made and put to MPs.

In a letter to ministers, Mrs May said colleagues with long-standing views or constituency interests on the issue would be given an "exceptional and limited" freedom to criticise the decision of the Cabinet's airports sub-committee, though they will not be allowed to campaign against it or call into question the process by which it was reached.

The letter also confirmed that the decision would be put to the House of Commons for a vote in the winter of 2017/18.

The choice on the Government's preferred option will be made not by the full Cabinet but by the airports sub-committee - which does not include Mr Johnson, Ms Greening or any other London MPs.