I reported last week on the remarkable intensity of red-on-red infighting over Brexit – between MPs, between factions, between non-Corbynites and Corbynites, and now even between different streams of Corbyn supporter. Last night’s Heathrow expansion vote offered an insight into what appears to be the growing willingness of parts of the Labour Party to openly defy their leader.

Labour had, reportedly, intended to whip its MPs to vote against the Heathrow plan – but was forced to grant a free vote by the blunt response that some scores of their MPs would simply defy such an instruction. Instead, they relied on making clear Corbyn and McDonnell’s own preferred policy. The Shadow Chancellor even spoke in his constituency capacity during the debate, opposing expansion in no uncertain terms. And yet he was only able to carry a minority of Labour MPs with him – 96 of them voted against, including tellers, and 119 voted for. That’s quite an extraordinary division, given their leader’s very public desire for them to vote with him.

Nor is the divide limited to the Parliamentary Labour Party. Just as we saw last week over Brexit, the factions weighed in, too. Len McCluskey and Unite backed the new runway, while Momentum opposed it, the former citing jobs, the latter deploying an environmentalist argument. The inherent tension between industrial growth and anti-growth green politics, which normally simmers away out of sight, went public – a handy reminder that for all the talk of solidarity and unity, there are fracture lines within the Opposition which can open up if pressed in the right way.

To an extent the division was about beliefs and ideology, as well as constituency interests. Heathrow’s campaigners have done a highly effective job in recent years of building a broad coalition of MPs across the country to counterbalance those in West London and parts of the South East who oppose it. But what an MP believes in principle, or what they think right for their constituency, is not always the dominant factor in how they vote – the forces of leadership, whips, power and patronage are often deployed to overcome those exact considerations, on a whole range of topics. What’s particularly interesting is the way in which so many Labour MPs felt able to first force a free vote and then to vote the opposite way to Corbyn, and with the Government.

That’s surely a sign of some dissolution of his authority over the PLP – and of diminished fear among Labour MPs of the consequences from the grassroots of being seen to disobey their leader. There are certainly recriminations underway – at least one Momentum branch has publicly attacked Labour MPs for failing to “take climate change seriously”, for example  – but either Opposition MPs are of the view that their bark is worse than their bite, or they think they are doomed anyway and therefore have nothing to be afraid of.

Intriguingly, there are more signs of this trend this afternoon. Barry Gardiner, the Shadow International Trade Secretary, chose to whip his colleagues to abstain on a motion supporting CETA, the EU-Canada trade agreement. PoliticsHome reports that the Labour Chief Whip “physically intimidated” MPs, but “was ignored” – with 15 MPs rebelling to vote in favour of CETA, and four others rebelling to vote against. That outcome calls to mind the three-way split on the Labour benches over the EEA amendment a fortnight ago. To perform such a feat once might be misfortune, but to do so twice in a few weeks suggests a deeper problem.

If such breakdowns of Opposition discipline are going to become more common, the question is what the consequences will be. It could be a good thing for the Government if it means they will benefit from rebellions in their favour, but it also makes for a nightmare for the Government whips, as tight votes will become harder to predict. For Labour, they might perhaps have less fear than before of the traditional assumption that voters don’t like divided parties – their well-publicised internal divisions likely mean that damage is already priced in to their polling numbers. If anything, their MPs might reason that if voters already believe them to be divided, then there is little harm in enjoying the liberation of actually dividing more often.