It is understood that the Prime Minister is very unlikely to back expansion at both London airports with a decision expected imminently.
The issue is highly politically sensitive given the opposition to a new Heathrow runway from Cabinet ministers like Boris Johnson and Justine Greening, and some backbenchers
A decision was repeatedly delayed by Mrs May’s predecessor David Cameron and Tories may be offered a free vote, or opponents given a leave of absence from the Commons when the issue comes up.
Tory MP Adam Afriyie said he would vote against Heathrow expansion in defiance of party whips if there was no free vote.
Asked whether he would oppose a third runway on BBC One’s Sunday Politics show, the Windsor MP replied: “Oh, 100% yes. I’m going to fight tooth and nail.”
Meanwhile, fellow Tory MP Kwasi Kwarteng said reports that there are 60 opponents within the party are “way off the mark”, telling the programme there are about “20 hardcore people”.
“I think it may well be a free vote,” he added.
It is believed Jeremy Corbyn is likely to offer Labour MPs a free vote if Mrs May plumps for Heathrow, with many in favour of a third runway in opposition to the leader and shadow chancellor John McDonnell.
Opposition MPs and peers are set to debate the issue at Monday’s meeting of the Parliamentary Labour Party (PLP), according to the Guardian.
On Monday, members of the Unite union will deliver a letter to 10 Downing Street signed by more than 125 reps, representing hundreds of thousands of workers, calling on the PM to approve Heathrow’s expansion.
The letter urges the Prime Minister to be “bold and build”, saying: “Giving the green light to Heathrow expansion will not only signal to the rest of the world that the UK is open for business post-Brexit, it is a big test of how serious the Government is about turning its soundbites on industrial strategy and infrastructure, backed by public investment, into reality.”
Unite is also calling on the Government to commit to using domestic manufacturers and steel to maximise the economic dividend and secure decent, well-paid jobs.
On Sunday, 50 MPs and representatives from the devolved assemblies in Scotland, Wales and Northern Ireland called on the PM to back expansion of the west London hub as it is “our gateway to the world”.
The group, which includes MPs such as Welsh former Cabinet minister Stephen Crabb, Northern Irish DUP deputy leader Nigel Dodds and Welsh Labour’s Stephen Kinnock, wrote in a letter to Mrs May: “Northern Ireland, Scotland and Wales all have our own successful and growing airports.
“But Heathrow is our gateway to the world, bringing tourists to our attractions and helping our exporters reach new markets.
“Expanding Heathrow will create nearly four times more jobs in our three nations than other options being considered.”
Mrs May’s Cabinet sub-committee on airports is due to meet very shortly, possibly as early as this week, to examine the arguments for the three options set out in last year’s Airports Commission report.
The Commission recommended in July 2015 that a third runway should be built at Heathrow.
Other short-listed options are extending the airport’s existing northern runway or building a second runway at Gatwick.
Downing Street said a decision will be made by the end of October.