The government is considering giving ministers a free vote on controversial plans to build a third runway at Heathrow, according to a document seen by Channel 4 News.

In the second security breach of its kind in a week, a London Underground passenger filmed a very senior Cabinet Office civil servant holding a paper that discussed “potential waiving of collective responsibility” ahead of “the forthcoming decision around airport capacity”.

The document discusses the possibility of “allowing Ministers to speak against the government’s position in the House” as they did in the EU referendum campaign.

If the vote goes ahead it could end decades of bitter argument over the best way to increase capacity in southeast England.

The issue has split the party in the past, with ministers like Boris Johnson and Justine Greening outspoken opponents of a new runway at Heathrow, while other Conservative MPs called for it to go ahead.

Most decisions taken at cabinet level are subject to the principle of collective agreement, where ministers are expected to publicly support government policy, even if they disagree with it in public.

But the leaked paper suggests this principle could be waived in this case, allowing ministers to vote according to their conscience and speak on both sides of the argument, like they did in the EU referendum.

The paper discusses following “the model of the recent EU referendum guidance”.

Crunch vote

The vote could cause difficulties for Theresa May, who may have to rely on Labour votes to get the legislation through parliament.

It is not yet clear what Labour’s official position would be in the event of a vote.

SNP leader Nicola Sturgeon has said her 56 Westminster MPs would decide the future of airport capacity in southeast England, but declared the party was “neutral” on whether expanding Heathrow or Gatwick was the better option.

Heathrow saga

Conservative support for Heathrow would mark a major reversal of policy.

Before the 2010 election, David Cameron memorably promised: “The third runway at Heathrow is not going ahead, no ifs, no buts.”

By 2015 the promise to stop the new runway had been dropped from the Conservative manifesto, and Mr Cameron was being attacked for delaying a decision.

Heathrow had emerged as the favourite after the independent Airports Commission backed a third runway at the west London hub, rejecting alternative suggestions like a new airport in the Thames Estuary.

Boris Johnson, then Mayor of London, had thrown his weight behind a scheme to build a new hub on an artificial island in the Thames in Kent, leading to the plan being nicknamed “Boris Island”.

Mr Cameron promised a decision on the airport question this summer, but Brexit forced him to resign and put the matter on hold.

Theresa May’s then promoted two of the most outspoken opponents of Heathrow expansion – Boris Johnson and Justine Greening – to senior ministerial posts, leading some to question whether the plan would go ahead.

Snapped on Tube

A Tube commuter spotted the email printout while standing close to the official on a Central Line train. The message was addressed to Sue Gray, director general of the Cabinet Office’s propriety and ethics team.

The passenger captured some of the contents with a smart phone, including a section clearly marked “waiving collective responsibility”.

This is the second time in a week that government officials have allowed sensitive documents to be photographed.

On Tuesday an official was snapped on his way into Downing Street holding a document on schools policy which revealed the government would “open new grammars”.

Channel 4 News has contacted Downing Street and is awaiting a response.