Leading Labour activists are warning Jeremy Corbyn that they could boycott the party’s campaign for the European elections unless it backs a confirmatory referendum on Brexit, as pressure mounts on the leadership to support a fresh public vote.

The warnings come before a crucial meeting on Tuesday of Labour’s deeply split national executive committee (NEC) at which the wording of the party’s European election manifesto is due to be decided.

The issue of whether to make an explicit commitment to a confirmatory referendum on any future Brexit deal in the European campaign is causing increasing tension across the party, with senior figures around Corbyn being accused of trying to backtrack on the policy, which has the clear backing of most members of the shadow cabinet.

Last week many Labour MPs and activists reacted with fury after a draft leaflet written by officials in Corbyn’s office, which aimed to introduce Labour’s European election campaign to voters, omitted any reference to another referendum. Protests have led to the leaflet having to be redrafted, according to senior party sources.

The Observer has also been told that hundreds of activists, councillors and supporters have written to the NEC and the leader’s office to demand that Corbyn stick to the referendum policy and promote it during the campaign – and many have said their support depends on his doing so.

I have had a lot of members tell me they won’t campaign for Labour unless the party clearly backs a new public vote Lynne Neagle, AM

On Saturday Paul Wilkinson, chairman of Gedling constituency Labour party (CLP) in Nottinghamshire, said he had let the party leadership know his position in the clearest terms. “I have been a party member for more than 30 years and have campaigned in every election that has taken place in that time. I am becoming increasingly concerned that the party is moving away from a position of supporting a confirmatory referendum for any deal agreed, with an option to remain,” he said. “I am horrified by the draft leaflet, which made no mention of a confirmatory referendum. I would find it very difficult, if not impossible, to campaign for such a position, which I would find heartbreaking.”

Anashua Davies, the secretary of Old Bexley and Sidcup CLP, said members in her area were leaving the party in droves over the issue: “As CLP secretary, I have been fielding resignation letters from members who have left the party in frustration that the leadership undermines its democratically agreed Brexit policy daily. There is no place for a Brexit Light party, only a Remain-and-reform alternative to the current fudge. Labour must be unequivocal about a second vote or deservedly risk the same sorry fate as the Tories.”

Lynne Neagle, the Labour member of the Welsh Assembly for Torfaen, added: “I have had a lot of members tell me they won’t campaign for Labour unless the party clearly backs a new public vote, and some have even told me they will consider using a tactical vote.” And Paul Richardson of Torfaen CLP said: “Lots of members are saying they won’t campaign unless we back a ‘people’s vote’ or a confirmatory referendum.”

On Saturday party sources said they expected the NEC would agree that a confirmatory vote would be included in the manifesto, but only as a safeguard to prevent a “bad Tory Brexit” deal being imposed on the country. Many Labour remainers want their party to be more explicit and to commit to another public vote on any Brexit deal involving the Tories.

The NEC is comprised of 40 members, including representatives from the shadow cabinet, MPs, trade unions and the party membership. The membership section is dominated by supporters and members of the grassroots movement Momentum, who will play a crucial role at the NEC on Tuesday. While most Momentum members back another referendum, the organisation does not have a policy on the issue.

Mike Buckley, the director of the campaign group Labour for a Public Vote, said Momentum’s leadership would be under huge pressure in the coming days from their own rank and file to instruct its delegates on the NEC to vote for a public vote. “If we fail to do so, we risk losing votes to the Lib Dems, Greens and Change UK, splitting the Remain vote and in all likelihood handing extra seats to Farage.”

James Hamilton, the chair of Bromley and Chislehurst CLP, said: “Our manifesto for the European election must reflect party policy, which I know has been reinterpreted over and over, but in fact it’s very clear to all who were actually involved in drafting it. We have to offer a public vote, with the option to Remain. People voting Labour in my area, in London, and indeed further afield will expect this.”

Labour opponents of a second referendum, including the party’s chair, Ian Lavery, say another public vote would anger Labour Leave supporters and damage the party’s chances at a general election.