The result hangs in the balance, with the government telling its own MPs they have to approve the third runway, while Labour is permitting a free vote on the issue. Scottish Nationalists, which had been expected to back the bill, are now said to be unconvinced about the benefits for Scotland.
Transport secretary Chris Grayling will introduce the bill at around 2.30pm. One government member, foreign secretary Boris Johnson, who had vowed to lie down in front of the bulldozers if the runway was approved, has chosen to be abroad rather than rebel.
If the third runway is approved, many years of legal challenges are likely to follow. Local residents will seek a judicial review over pollution issues and the loss of hundreds of homes, while airlines will want reassurance that airport fees are not increased dramatically to pay for the project.