(Adds Heathrow comment)
By Kylie MacLellan
LONDON, Oct 18 (Reuters) - Britain will make a long-awaited decision next week on where to allow airport expansion near London, Prime Minister Theresa May's spokeswoman said on Tuesday, with ministers who oppose it given the rare opportunity to publicly voice their dissent.
The decision will be subject to public consultation before it is put to parliament for its backing late 2017 or early 2018.
The government will either support plans for a new runway west of London at Heathrow, the busiest airport in both Britain and Europe, or at Gatwick to the south. The decision has been debated for more than 25 years.
May's cabinet discussed the options for more than an hour on Tuesday, her spokeswoman said, but the decision will be made by a smaller group of ministers.
"It was not for cabinet to take a decision today. That will be taken by the cabinet sub-committee on airports, which is due to meet next week and will select a preferred option," the spokeswoman told reporters.
"There was a strong consensus about the importance of this decision for the country, the need to get on and make a decision ... and what it would mean for the UK economy and the nation that we want to be as we leave the European Union."
The sub-committee, chaired by May, does not include key opponents of Heathrow expansion such as Foreign Secretary Boris Johnson and Education Secretary Justine Greening.
Media reports have said it will meet next Tuesday, Oct. 25, but the spokeswoman declined to specify the date.
In 2015 a three-year independent inquiry set up by the government recommended building a new runway at Heathrow, subject to a list of conditions over night flights, noise and air quality, and it is seen as the front-runner.
Heathrow said in a statement the process was in line with its expectations, and that if given the go-ahead it would apply for planning approval following the parliamentary vote.
In a break with normal procedure that ministers must support the government position, May took the rare step of promising colleagues who oppose the decision that they would be allowed to publicly air their views.
Media interpreted this as a strong signal the government would back Heathrow, as there are no high profile ministers who oppose Gatwick expansion.
In a letter to ministers published on Tuesday, May said the exception would only apply to those who had previously expressed strong opinions or had a directly affected constituency.
Ministers would have to seek her approval to oppose the government and would not be able to actively campaign against the decision, speak against the government in parliament or criticise the decision-making process itself, she said. (Additional reporting by William James, Editing by Stephen Addison)