Labour is facing its most fraught annual conference in 15 years as a bitter row about the party’s position on Brexit comes to a head in Liverpool this week.

After a summer on the back foot because of allegations of anti-Semitism, Jeremy Corbyn’s party is braced for another explosive argument about their opposition to holding a second referendum on whether to leave the European Union. Many in the rank and file want to use this week’s party conference to push for a change of policy — and insiders warn that the showdown could be the most rancorous since Labour was divided by the Iraq War.

Days after Theresa May’s Brexit policy was humiliatingly rejected by EU leaders at a summit in Salzburg, Austria — setting off another round of bickering among Conservatives and a new debate about May’s leadership — the Labour conference will make painfully obvious that the opposition, too, is struggling to come up with a coherent plan for leaving the union.

It won’t just be the usual centrist, Remain-supporting MPs hammering away at their leader. The Brexit fissures run deep into Labour’s Corbynite wing, dividing the trade unions and the members from whom the leader derives his power. How these dynamics play out in the next few days — and the decisions that Corbyn makes as a result — could have a significant impact on the final few months of the withdrawal process and the shape of Britain’s future relationship with the EU.

But it’s more than just a debate about Brexit policy, party insiders say.

The argument about the second referendum is indicative of a backroom struggle for control of Labour that has been building for months and, the insiders say, will profoundly affect the direction of the party’s policies and its political decision-making.

“You’ve got four or five different camps pulling in different ways, on a lot of different issues,” said one trade union source.

Media coverage of Labour tends to dwell on opposition to Corbyn from centrist MPs, but insiders say that it’s the developing tensions on the left that are most worth watching.

Unions that tended to speak as one bloc under previous leaders are now increasingly fragmented, including on Brexit. On the other side, there’s an increasingly assertive membership, led by the grassroots activists Momentum, who are pushing the party to become more radical than even Corbyn wants to be.

This is not to say Corbyn’s leadership is at risk. In some ways, he has more say over the party than other Labour leaders ever had, with control of most of its internal levers. Most, if not all, of the competing players in this developing struggle are supportive of Corbyn himself, and broadly aligned with his political vision. But the new power dynamics are a headache for the Labour leader, and his inner circle have spent a lot of time in recent months working behind the scenes to manage the tensions.

The backroom fight is awkward at a time when some of Corbyn’s lieutenants — notably the shadow chancellor John McDonnell — have been urging Labour to temper its radicalism to enhance its electoral appeal. Anticipating a general election if the Conservatives collapse because of Brexit, McDonnell has been trying to convince his colleagues that Labour now needs to project itself as a credible, united political alternative to the Tories.

Among those angling for influence over Corbyn are the major trade unions, which remain important players in his firmament. They’re still a major source of funding for the party. And Corbyn himself, more than any other recent leader, is a staunch unionist.

None is more powerful than Unite. Its general secretary Len McCluskey is close to Corbyn and was crucial to Corbyn consolidating his position as leader. The union is a major financial contributor, giving more than £6.6 million to Labour since the start of last year. Its ties to the top of the party were cemented in April when Jennie Formby, Unite’s political director, became Labour’s general secretary. “Unite are obviously bigger than [other trade unions] and they do have a lot more influence,” one union source said.