Ministers could be given a free vote on whether to expand Heathrow Airport, an internal Government document has revealed.

The paper, which was filmed by a passenger on the London Underground, discusses the "potential waiving of collective responsibility" ahead of the forthcoming decision on airport capacity.

The long-awaited decision on whether to expand Heathrow or Gatwick is politically highly sensitive for Theresa May due to divisions within the Tory ranks.

Foreign Secretary Boris Johnson campaigned against Heathrow expansion, while London Mayor Sadiq Khan and Putney MP and Education Secretary Justine Greening are also opposed.

Details of the proposal were contained in an email sent to Sue Gray, the director general of the Cabinet Office's propriety and ethics team, Channel 4 News reported.

A Tube commuter spotted the email printout while standing close to the official on a Central Line train.

The passenger captured some of the contents with a smartphone, including a section clearly marked "waiving collective responsibility" and appearing to indicate that one option would be a free vote.

The disclosure is the second embarrassing document leak to hit the Government within days after the Prime Minister's plans for new grammar schools were inadvertently revealed to press photographers in Downing Street.