A key adviser to Jeremy Corbyn has reportedly quit saying the Labour leader cannot win the next general election.

In a significant blow to Corbyn, Andrew Fisher, who masterminded the party’s 2017 manifesto, said he no longer had faith that Labour would be successful.

Fisher wrote a memo to colleagues, the Sunday Times reports, saying members of Corbyn’s team had a “lack of professionalism, competence and human decency”. He also accused them of making a “blizzard of lies and excuses” and apparently claimed that the highest ranks of the party were engaged in “class war”. Labour said it did not comment on staffing matters. Fisher said in a statement reported by Sky News that he wanted to spend more time with his family and would remain until after the likely autumn general election.

The 40-year-old has been a controversial figure within the Labour movement. He was suspended from Labour in 2015 for apparently supporting a Class War candidate against Emily Benn, Tony Benn’s granddaughter, in the general election, and Benn called for him to be expelled. He also appeared in a video saying he had “very violent, bloody nightmares” about hitting former Labour cabinet minister James Purnell.

Barely a year after his suspension, Fisher was confirmed as Labour’s executive director of policy, and was given a large share of the credit for Labour’s surprise success in the 2017 general election which saw Theresa May lose her majority.

Fisher’s resignation adds to bitter infighting that has dogged the party. It boiled over again this weekend when pro-Remain Labour MPs and activists accused Corbyn of trying to “shut down democratic debate” on Brexit. The row, which broke out after the leadership tabled plans to delay a decision on whether it should back Remain or Leave until after a general election, came as a new Opinium poll for the Observer showed the Tory party had extended its lead over Labour to 15 points.

Labour activists hand out bags ahead of the 2019 Labour party conference in Brighton. Photograph: Dan Kitwood/Getty Images

The survey put the Conservatives on 37% (unchanged from two weeks ago) with Labour on 22% (down three percentage points) , the Liberal Democrats up by one point on 17% and the Brexit party down one on 12%.

Also worrying for Labour, before a probable general election in which Brexit would be certain to dominate, the survey found that almost seven out of 10 voters (69%) now believe Corbyn’s policy on Brexit – to back a second referendum while not recommending either Leave or Remain – is unclear.

In contrast, well over half of voters think the Conservatives (58%), the Liberal Democrats (59%) and the Brexit party (70%) have clear Brexit policies. Opinium also found for the first time that Remain voters were now just as likely to vote for the Liberal Democrats as for Labour. The Lib Dems backed a policy of scrapping Brexit if they formed the next government at their conference in Bournemouth last week.

Ninety constituency Labour parties (CLPs) have submitted motions to Labour’s Brighton conference on Brexit, 90% of which back a clear Remain position. The motions reflect a widespread belief among activists that the lack of a clear policy is driving away voters and would be fatal to the party’s chances at a general election.

On Sunday evening shadow Brexit secretary Keir Starmer is expected to attend a key meeting at which the Brexit motions from CLPs will be combined into one, before being debated and voted upon on the conference floor on Monday.

However, the process was thrown into confusion on Saturday when Corbyn tabled a draft statement to the party’s national executive committee (NEC) which said the crucial decision on how Labour would campaign in a referendum would be put off until a “one-day special conference, following the election of a Labour government”. The move was seen as an attempt to override the pro-Remain motion and prevent it being debated and voted on. While Corbyn now accepts that Labour should pursue a referendum in an election campaign, he also fears driving away Labour supporters who backed leaving the EU, and has indicated that he would want to stay neutral when a second vote happened – a position regarded as unsustainable by many in the party.

A member of the NEC who was present said the statement met “serious objections” from several NEC members, and some unions, and was not passed because of widespread demands for amendments. The NEC will meet again on Sunday to discuss the statement a second time.

But when its content was made public, Remain activists and MPs were appalled, saying the party was trying to use the NEC to crush democratic debate and silence the views of a clear majority of party members.

Clive Lewis MP, a shadow Treasury minister, said: “This move is just plain wrong. How can this be defended? We, the left, took over the leadership of this party promising internal democracy, promising a new kind of politics. And yet here we are, with a leadership apparently determined to shut down democratic debate on the crucial issue of the day, probably relying on union bloc votes to outvote the members. It’s not what we signed up for. We now need to rally on the conference floor – if it passes, delegates should mobilise to vote against the NEC statement so the Brexit motions can be heard and democratically debated.”

Michael Chessum, national organiser for Another Europe is Possible, the organisation that has championed the pro-Remain motion, said: “Introducing an NEC statement in this manner would be a bare-faced attempt to shut down a democratic debate on Brexit at conference. The idea that Labour would not take a position now, and put it off to a special conference just after an election, is absurd. We have a conference, right here and right now, which has had a huge wave of grassroots motions submitted to it on Brexit. Ahead of an election, Labour’s members and voters want to know that the party understands them and is on their side. Preventing their views from even being debated is just a slap in the face. The NEC should vote down this statement and let the members decide our policy democratically on the conference floor.”

Lloyd Russell-Moyle, Labour MP for Brighton Kemptown, said: “We are being hammered on the doorstep because our Brexit position is a fudge. Yes, it’s great that we are putting forward a public vote and a Remain option. But in every seat in the country, Leave and Remain, we are losing votes because our voters are turning to Remain parties. 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.”

In an interview on the eve of conference, Corbyn suggested he would campaign to remain in the EU if that was what the conference decided. “I am the servant of the people and the party,” he said, while stopping short of explicitly backing a Remain position.

Separate polling this weekend by YouGov for the People’s Vote Campaign showed 46% of voters thought that any effort by a party leader to “stay neutral” in a Brexit referendum would be “unreasonable” – only 26% thought it acceptable.