On Monday evening MPs will vote on whether or not Heathrow airport should have a third runway. It is a deeply factious issue and, not unexpectedly, the divisions are complex.

Conservatives – mainly for

The Tories have imposed a three-line whip – the highest level of party discipline – to ensure the party’s MPs back the transport secretary, Chris Grayling. The vast majority will, but with a few high-profile exceptions.

Justine Greening, the former education secretary, and Zac Goldsmith both have west London constituencies and are avowed opponents of the scheme.

Goldsmith famously resigned from his Richmond Park seat in 2016 in protest at the plan, then lost in the subsequent byelection to the Liberal Democrats and had to wait until the 2017 general election to return to parliament.

The biggest controversy circles, as ever, around Boris Johnson. The foreign secretary represents Uxbridge, right in the airport’s flight path, and once vowed to “lie down … in front of the bulldozers” if necessary.

However, while Greg Hands resigned last week as a trade minister to vote against the government, Johnson has stayed silent, and is expected to be abroad at the time of the vote, thus avoiding difficult decisions – something which has enraged some of his fellow Tory MPs.

Labour – a mix

The party’s official policy is to oppose the expansion and, indeed, when it was in the news last year it was heavily trailed that it would do so. However, while many Labour MPs are against the plan, there is another factor at play: Unite.

The union represents more than 34,000 workers at the airport. Its head, Len McCluskey, has written to all Labour MPs urging them to back the expansion plans.

The result is that party whips are offering its MPs a free vote, and anything from around 40 to nearer 100 are expected to back the government. This could see a series of shadow cabinet members going against Jeremy Corbyn. Those tipped to do so include Angela Rayner, Nick Brown, Barry Gardiner, Dawn Butler and Ian Lavery.

Scottish National party – was for, now against

The SNP government in Scotland has been supportive of the Heathrow plan, arguing that it would benefit the Scottish economy. As such, the party’s 35 representatives in Westminster were expected to back the move.

However, it is widely reported that the SNP is cooling on the idea, officially over doubts about the merits of the expansion, though possibly also to avoid the symbolism of voting with the government.

Liberal Democrats – against

The party’s 12 MPs are expected to line up firmly against the plan, not least as the Twickenham constituency of their leader, Vince Cable, is directly in the airport’s flight path.

Other parties

Democratic Unionist MPs are connected to the government through the informal coalition, and support the expansion plan anyway. Plaid Cymru have backed a bigger Heathrow in the past, on the condition that rail links to Wales are improved.

The sole Green MP, Caroline Lucas, has been one of the most vehement critics of the scheme, calling it a huge environmental step back.