Phil Wilson, an influential Labour MP, told The Independent: “Conference will attempt to make sure the party backs Remain and that the leader does as well.”
And grassroots activists pointed out that no fewer than 90 per cent of motions put forward by local parties call on Labour to campaign to stay in the EU, rather than stay neutral.
“Support for an explicit Remain stance is evidently overwhelming,” said Michael Chessum, the national organiser for the Another Europe is Possible group.
The backlash came as the Labour first minister of Wales made clear Mr Corbyn’s plans to avoid picking a side would be ignored by the party there.
“In that referendum, we, as Welsh Labour, must and will campaign to Remain in the EU,” Mark Drakeford wrote in a letter to party members.
The criticism sets the scene for a repeat of the marathon five-hour “compositing” motion at last year’s Labour conference, which ended in Brexit policy being fudged.
This year, in Brighton, Mr Corbyn will hope the support of the big trade unions will enable his policy to win out in the battle with the overwhelmingly pro-Remain local parties.
He then wrote: “We would then put that to a public vote alongside Remain. I will pledge to carry out whatever the people decide, as a Labour prime minister.”
The comment was immediately interpreted as an intention to remain above the fray by staying neutral, while allowing Labour ministers and MPs to campaign for whichever side they chose.
In a TV interview, Mr Corbyn repeatedly declined to say which side he would back in a second referendum, again hinting he would remain neutral.
And he risked angering Labour MPs and members by arguing the Leave option he would put forward “would protect jobs and living standards and trade”.
Pressed if he would remain neutral in the campaign, Mr Corbyn replied: “As prime minister I'm offering the people a choice – the only party that's doing so.”
“I'm asking my party conference this weekend and next week in Brighton, and all my colleagues, to realise the importance of giving the people a choice,” Mr Corbyn said.
“I've shared these views with all of my colleagues in the party and I'm very confident they will come with me on this journey to make sure that the people of this country make the final decision.”
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