There is a truism in politics that “when you decide, you divide”. The row about debating Brexit openly at the Labour party conference this year has shown that this is one of the new realities facing Jeremy Corbyn, as it becomes apparent that the creative fudge that got the party through the 2017 general election won’t get Labour into government.

The process that gives us a glimpse of it this week may be niche, but it foreshadows the broader generational struggle Corbyn now faces, between his Bennite fellow travellers – committed Eurosceptics for decades – and the largely pro-European crowds that cheered him at Glastonbury and voted Labour in June. It is a divide that threatens Corbyn’s large electoral coalition. What we have learned is that, for all the focus on innovative online activism and grassroots organisation, the leadership of Momentum, the pro-Corbyn campaign group, remains true to its ideological Bennite roots.

Like many others, I woke up on Sunday to an email from allies of the Momentum leadership, whose chair, Jon Lansman, cut his teeth running Tony Benn’s deputy leadership campaign in the 1980s on an anti-European message. The morning briefings for Labour party conference delegates included recommendations for the priorities ballot – Labour’s process for sifting a huge number of motions to just four picked by the unions and four by the constituency Labour parties.

Momentum members were told to pick the NHS, housing, social care and rail – all very worthy, and the source of little real contention in the party. They were informed that some areas – workers’ rights, investment and growth, public sector pay and Grenfell – did not need to be voted for because the unions had “already agreed to prioritise” those debates. On the most important issue facing the country, Brexit, delegates were told not to worry as the issue was “already set to be debated on the Monday morning conference session”.

This was a blatant distortion of what would follow. Brexit not being prioritised meant Momentum robbed the delegates – and party members at large – of having any democratic input on Brexit, the single market and free movement. They had the chance to applaud shadow Brexit secretary Keir Starmer on a well-executed speech and its content on Monday morning, but not the opportunity to shape what Labour’s policy might be.

Many pro-Momentum delegates voted against debating Brexit because they had faith in the line they had been given. They trusted Lansman and those in charge of Momentum. But they have been let down by those who wanted to disguise the gulf between the beliefs of the movement’s figurehead, and the beliefs of the masses who make up the movement. There will now be no vote on Brexit at this conference – a ludicrous state of affairs. How can Labour claim to be a government-in-waiting if it dodges the biggest issues of the time and actively fears the debate within its own party? It does not bode well.

Thousands of new party members over the past two years, millions of new voters in June and the vast majority of Labour MPs are passionately pro-European. The delegates on the conference floor only rejected the topic because they trust the Momentum leadership and followed their instructions. That leadership position is dishonest. It is driven on this issue, like the majority of the leader’s policies, from a longstanding leftwing Euroscepticism. Those able to vote in 1975 opted to keep Britain out, those who have been in parliament in the intervening years took anti-EU positions. Who knows how many may have used the secrecy of the ballot to vote leave last June?

Either way the clarion call that this conference is about the voice of the members has proved to be a sham, and the Corbyn voters of 2017 will see this as the same old politics of broken promises. The reality is that those who made up the “Corbyn surge” were more likely to be middle-class, university educated, urban and pro-European, despite what Labour’s manifesto actually said on the subject. Corbyn had a choice. Did he side with his Bennite comrades of old or the Corbynite crowd at Glastonbury?

On that question, how Labour’s new establishment have used the stitch and fix at this conference might have revealed all we need to know – and the Labour leader’s creative fudge during the election now leaves a rather bitter aftertaste. Currently, the coalition is being held together by their shared loyalty to the Labour leader. That has allowed him the benefit of the doubt – for now. But the decisions required to show that the party really is a government-in-waiting cannot be dodged. Eventually Corbyn will need to show which side of the divide he is on.

• Richard Angell is director of Progress – Labour’s centrist pressure group