Emily Thornberry, the shadow foreign secretary, told activists that Labour should campaign for Remain “whatever terms are agreed" with the EU, while Sir Keir Starmer, the influential shadow Brexit secretary, said it was "very important" to listen to pro-EU party members.
They spoke out as MPs and grassroots activists dismissed a compromise plan proposed by Labour's ruling executive as a "stitch-up" and said they would push ahead with plans to force Mr Corbyn to back Remain.
Addressing a People's Vote rally in Brighton, in front of what organisers said was 5,000 people, Ms Thornberry insisted that "the members must be heard, because this is what democracy looks like".
Wearing an EU flag, she said: "Whatever terms are agreed by which we leave the European Union, by whatever government, no matter what it says, we must make sure that there's a second referendum, we must make sure that Remain is on the ballot paper, we must make sure that Labour campaigns for Remain - and not just that, but that we lead the campaign to Remain."
She added: "We believe in internationalism. We believe in socialism. And if we believe in internationalism and socialism - why on earth would we back Brexit?...We all say no, no, no to Brexit."
Citing reports that some members of the shadow cabinet wanted to campaign for Remain, she said: "No s*** Sherlock. Yes we're campaigning to Remain."
Ms Thornberry said it would be a gift to the Liberal Democrats and SNP if Labour pledged to deliver Brexit, and criticised suggestions that anti-Brexit voices in the party were undermining Mr Corbyn.
She continued: "When people have taken a position that we hate Brexit, that we wish to remain and reform the EU, don't you dare question our commitment to socialism and don't you dare question our loyalty to Jeremy Corbyn.
"It's because of our commitment to socialism and because of our commitment to Jeremy Corbyn that we are here today demanding an outcome that will give Jeremy Corbyn the best chance possible of getting into Number 10."
Her mention of the Labour leader was greeted with boos from some sections of the crowd, along with chants of "Corbyn Out" and "Where's Corbyn?"
Sir Keir also argued that Labour had "got to listen" to pro-EU party members and confirmed that he would campaign for Remain in any referendum.
Speaking at his first People's Vote rally, the influential shadow Brexit secretary said he was now "utterly convinced" that a referendum was the only solution to the Brexit crisis.
He said: "We've come a long way. This time last year we were talking about keeping the option of a People's Vote on the table and now the Labour Party has said whatever the outcome, whether its under this government or the next, it's got to go back to the public to decide a basic question: are you prepared to leave on the terms on offer or do you want to remain?
"When that time comes I will campaign for Remain alongside millions of other people in this country, because it's not just a technical question of whether you want to be in or out of the EU, it's about what sort of country we want to be."
He added: "So my message is very simple: if you want a referendum, vote Labour. If you want a Final Say, vote Labour. If you want to make the case for Remain, vote Labour and let's get on with it."
Speaking afterwards, Sir Keir said he wanted to "work with members" to find an agreed position, adding: "I think it's very important that members are heard in this - that's why we have conference...It's undoubtedly the case that the members want to have a view on this, do have a view on this and want to be heard on this."
He told The Independent: "I think any polling of the party and any engagement with the members demonstrates that a majority of members want to back Remain and therefore we've got to listen to what they've got to say about it, but we do have to find a way forward that everybody is comfortable with."
Despite 90 per cent of motions submitted to the conference by local Labour parties backing Remain, the NEC proposed that a Labour government should delay adopting any position in a possible second Brexit referendum until it had tried to negotiate a new exit deal with the EU.
Labour MP Lloyd Russell-Moyle said: “This conference is our one chance before an election to get out of the fudge – we cannot allow that to be taken away from us in some procedural stitch up.”
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