At the start of the week, Labour’s strategy team – Jeremy Corbyn, his aides, plus Diane Abbott, the shadow home secretary, John McDonnell, the shadow chancellor, plus their aides – attended a polling presentation by BMG Research, the party’s new pollster.

It made for grim viewing according to one attendee. Among the horror stories: for two thirds of voters, their vote in the referendum is now more important than their chosen political party. For Labour the effect is particularly deadly. Their voters largely backed Remain, but that segment is concentrated in a handful of safe seats. Overall, many Labour seats trend Leave. Separately, the leadership is braced to lose both the Copeland and Stoke-on-Trent by-elections.

Labour is effectively led by a triumvirate – Corbyn, plus Abbott and McDonnell, the two MPs who are most influential upon his thinking – though Emily Thornberry, the shadow foreign secretary is growing in importance. Unlike Abbott and McDonnell, she is not a longstanding ally but was the only MP to make the leap from opposing Corbyn in the 2015 leadership race to supporting him in the 2016 one. As Corbyn’s constituency neighbour since 2005, she and him get on well and hers is one of the voices that is listened to closely.

The presentation sold Corbyn on the idea that Labour had to show that it respects the result of the referendum if it is to pass through this period of uncertainty. Abbott and Thornberry – whose own seats, like Corbyn’s, voted to Remain by a large margin – are less sold on the new approach. McDonnell, however, was deeply affected by presentation, which helped to stiffen the Labour leader’s resolve.

That was behind the decision to impose a three-line-whip as far as voting for Article 50: to show that Labour has accepted the result and help drain the poison from the issue. That Corbyn is a longstanding Eurosceptic, who has voted against every European treaty that came before the House during his parliamentary career, means that it is less of a wrench for him than for his other colleagues.

While there are private disagreements about the new line among the top four, that quartet were largely united in shadow cabinet. It was Clive Lewis who spoke out. Although he is not as influential in the inner circle as is widely supposed, he is regarded as the best hope for the Corbynite wing to retain control of the party after the next election. So it is significant that he went so far as to hint he might resign, but “stopped short”, in the words of one present. He will now vote with his leader to trigger Article 50.

What happens now? Although the party’s three-line-whip would usually result in a sacking if the whip is broken, it is likely to be widely breached without serious consequences. That Daniel Zeichner, Tulip Siddiq and Catherine West have all committed to vote against triggering Article 50 (and in the case of Zeichner and West in particular they would seriously imperil their seats if they didn’t) but are judged to have been helpful presences on the frontbench mean that the leadership might be inclined to “look the other way”, or at the least, restore them to the frontbench in short order. Siddiq and Keir Starmer, the shadow Brexit secretary, both broke a three-line whip on HS2, which the leadership supports but their constituents oppose, with Corbyn intervening to preserve both of them.

Note: At time of publication, I misread a message about the composition of the strategy meeting, and added Emily Thornberry to the attendance list. The fault was mine, apologies.