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The government will next week announce its decision on whether to favour expanding either Heathrow or Gatwick airport, after decades of delays.

Unusually, the decision will not be taken by the full cabinet but by a sub-committee, chaired by Theresa May.

MPs will not get to vote on the decision for at least another year.

Some ministers will be allowed to speak out against it for a limited period in a move being seen as evidence a third runway at Heathrow will be backed.

Expanding Heathrow is strongly opposed by Foreign Secretary Boris Johnson, and Education Secretary Justine Greening.

Allowing ministers to speak out could avert the possibility of resignations from cabinet.

In a letter, Prime Minister Theresa May has told cabinet colleagues that once a decision has been taken by the airports sub-committee on the preferred scheme it will then be subject to a "full and fair public consultation" before a final decision is put before the Commons in the winter of 2017-18.

Number 10 would not comment as to whether MPs would be able to vote freely on the matter.

A Heathrow spokesperson said it was "the expected and appropriate political process" - a view echoed also by Gatwick.

Analysis: Richard Westcott, transport correspondent

The government, the airports - they all insist that this is not a delay and that it was always an expected part of the planning process.

Ministers will still pick a winner next week, they say. We think they will plump for a third runway at Heathrow.

MPs might then get the chance to vote on the issue quite soon. However, it will not be binding.

It will just be a chance for everyone to air their views, including MPs such as Boris Johnson and Justine Greening, who are both vehemently opposed to expanding Heathrow.

Critically, though, the binding MPs' vote - the one that counts - will not happen for another year or so.

Earlier, Mrs May told ministers at a cabinet meeting that a decision on increasing airport capacity in the South East had been "delayed for too long".

Her spokeswoman said the prime minister believed it was important to now take a decision "in the national interest".

The nine members of the airports sub-committee do not include Mr Johnson, whose Uxbridge and South Ruislip seat is close to Heathrow, Putney MP Justine Greening or any other minister representing a London constituency.

Mrs May's spokeswoman said the decision to give ministers a limited period to voice their personal views was a "mature, common-sense approach reflecting the fact that many ministers have long-held views and that ministers are also MPs and some have specific constituency issues that they have to address".

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As many as 60 Tory backbenchers could vote against expansion at Heathrow, where options include building a third runway, or lengthening one of the existing runways.

Zac Goldsmith, the Tory MP for Richmond Park, has vowed to resign from the Commons if the government approves a Heathrow expansion.

The Evening Standard reported on Tuesday that the local Conservative party would back Mr Goldsmith if he stood for re-election as an independent.

Airlines and business groups favour expansion of Heathrow, which offers far more direct connections than Gatwick and handles much more freight.

A final decision on which London airport to expand has been years in the making.

In 2009, former prime minister David Cameron pledged that there would be no new runway at Heathrow.

In July 2015, the Airports Commission chaired by Sir Howard Davies backed a new third runway at Heathrow, but did not rule out the option of expanding Gatwick.

Mr Cameron had promised a decision by the end of last year on whether to build a new runway at Heathrow.