Jeremy Corbyn was struggling to contain an open revolt by some of his most senior shadow ministers, MPs and party activists last night as anger over his refusal to back a policy of remaining in the EU threatened to wreck the Labour conference.

With delegates already reeling from a failed attempt by Corbyn supporters to oust Tom Watson and abolish his role as deputy leader, anger erupted amid accusations that the leadership was trying to block democratic debate and fudge a decision about where Labour stands on the issue of leaving the EU.

What was supposed to be a conference to showcase a party united behind new policies on education and health before a likely general election instead opened amid bitterness and acrimony, with a defiant Watson still in place, and Emily Thornberry, Keir Starmer and Clive Lewis, the shadow foreign secretary, Brexit secretary and Treasury minister, publicly defying Corbyn by backing Remain.

Leading a march through the streets of Brighton in favour of a people’s vote yesterday, hours after Corbyn had tabled a motion to the national executive committee (NEC) in favour of delaying a decision on whether to back Remain or Leave, Thornberry said: “We have got to campaign to remain. We have got to stop messing around.”

At the same event Starmer insisted that he would back Remain because “it’s about what sort of country you want to be”, while Lewis accused Corbyn of trying to use union block votes to stifle the views of the mass membership which had propelled him to the leadership in the first place. Their defiance came as news emerged that Labour’s head of policy, Andrew Fisher, a key Corbyn aide who masterminded the 2017 election campaign, had resigned, reportedly telling colleagues he did not believe the party could win a general election. In a memo to colleagues, Fisher is said to have accused Corbyn’s team of a “lack of professionalism, competence and human decency”.

A leading leftwing activist, Michael Chessum, who has coordinated local party efforts to push a pro-Remain motion through conference, told the Observer that patience among delegates was running out as the leadership appeared intent on defying ordinary members and imposing central control. “There is a really surreal edge to the atmosphere, partly exacerbated by attempts to delete Tom Watson – but also on Brexit,” he said.

“We have a party that wants Remain, voters overwhelmingly wanting clarity, MPs that want Remain and a front bench that wants Remain – and yet a ludicrous insistence that the party machine might not back them.”

The conference is now heading for a series of flashpoint moments. On Sunday there will be key discussions on what Brexit motion goes forward for debate and a vote on the conference floor on Monday. Then on Tuesday Watson is planning to make his own call for unity in his deputy leader’s speech, before Corbyn’s keynote address on Wednesday.

Tom Watson, the deputy Labour leader, has criticised Jeremy Corbyn’s Brexit stance. Photograph: Leon Neal/Getty Images

The latest Opinium/Observer poll today shows that Labour (on 22%) has now fallen 15 points behind the Tories (on 37%) despite Boris Johnson’s turbulent start, while the hardline pro-Remain Liberal Democrats are on 17% and the Brexit party on 12%.

Alarmingly for Labour, 58% of those polled now think the Tories have a clear policy on Brexit, against just 31% of voters who say Labour’s approach is clear. The poll also shows for the first time that as many Remain voters (33%) now plan to vote for the Liberal Democrats as for Labour. In April the proportion planning to vote Labour was nearer 50%, with only around 10% saying they would choose the Lib Dems.

Yesterday’s ructions over Brexit broke out after it emerged that Corbyn had tabled a statement to the NEC saying that, while the party backed a referendum and would offer the options of a credible Brexit deal or Remain to voters, a decision on how it would campaign in a second public vote would be left until after a general election. Pro-Remain activists and senior party figures who have campaigned for months to shift policy were outraged. They saw the move as an attempt to kill off debate and block a conference vote on their own pro-Remain motion.

Last night, however, the leader’s office appeared to back off. It told delegates that the pro-Remain motion would not be superseded by the NEC statement, and would still be debated. It now appears that two competing votes – one on the NEC statement, and the other explicitly backing Remain – could be debated and voted on on Tuesday.

Asked yesterday if he had known about the plan to oust Watson, put forward to the NEC on Friday by Jon Lansman, the founder of Momentum, Corbyn swerved the question. Then after an outcry, and protests from senior figures including the former party leaders Tony Blair and Ed Miliband, Corbyn said that the plan would not proceed and would be replaced by a review into the role of the deputy leader.

Watson, who is understood to think Corbyn was fully aware, described the attempt to oust him as a “drive-by shooting”, adding in a BBC interview: “I got a text message in a Chinese restaurant in Manchester to say that they were abolishing me. It’s a straight sectarian attack on a broad-church party and it’s moving us into a different kind of institution where pluralism isn’t tolerated, where factional observance has to be adhered to completely and it kind of completely goes against the sort of traditions that the Labour party has had for 100 years.”