Jeremy Corbyn has come under growing internal pressure to commit Labour to a unequivocal policy of remaining in the European Union as more than 100 councillors issued a joint warning to the party’s ruling body that any form of Brexit would threaten jobs, public services, workers’ rights and the environment.

In a letter to the national executive committee (NEC), which meets this week, the Labour councillors, including several leaders of county and borough councils, called on the party “to campaign unambiguously and energetically for a public vote on Brexit and to endorse a ‘remain and transform’ position in all circumstances”.

The group also said it would “support revoking article 50, if necessary to prevent no deal, and a commitment to a remain and transform position in a general election Labour manifesto”.

The grassroots intervention comes ahead of Labour’s annual conference starting in Brighton next weekend, at which the party’s stance on Brexit will be the subject of heated debate.

With the Tories and Brexit party committed to leaving the EU, and Liberal Democrats, SNP and Greens firmly in favour of remaining, many Labour MPs, members and activists believe the party needs a similarly clear Brexit policy on which to fight a general election. Labour’s current position is that it would try to renegotiate a new Brexit deal if it formed a government, then put that to a second referendum with the options on the ballot paper being to remain in the EU or accept the new deal.

Emily Thornberry is among shadow cabinet members who said they would vote to remain. Photograph: Tolga Akmen/AFP/Getty Images

Several members of Corbyn’s shadow cabinet, including the shadow foreign secretary, Emily Thornberry, the shadow chancellor, John McDonnell, and the chief whip, Nick Brown, have all said they would vote to remain in such a referendum, rather than vote for their own party’s deal. Corbyn has not said how he would campaign or whether the party would officially take a side.

While the nuanced position aims to accommodate both Remain and Leave voters, and is backed by several unions, there are signs of a growing grassroots revolt in favour of an unambiguous commitment to remain. Motions to the conference have already been submitted by more than 60 local branches calling on Corbyn to drop his plan to renegotiate and to commit to staying in the EU in all circumstances. The NEC will meet on Tuesday to discuss Brexit and how to handle the issue at conference.

Last year, the subject dominated Labour’s conference in Liverpool. Eventually the party emerged with a fudged position under which it agreed to keep the idea of a second referendum on the table, if it could not force an early general election.

The 2018 Labour conference in Liverpool fudged the Brexit issue. Photograph: Will Oliver/EPA

In their letter to the NEC, the councillors say: “As Labour councillors we struggle with huge difficulties caused by Tory government cuts. Further, the economic effects of Brexit have already, and will continue, to increase the problems our constituents face and the barriers councils face in delivering local public services, including fighting the climate emergency.

“Any form of Brexit threatens jobs, workers’ rights, migrants, the NHS, public health, public services and the environment, and will make it harder to deliver a radical Labour manifesto. Ending austerity, expanding common ownership, fixing the social care system, and delivering a national education service will be hindered by any form of Brexit. If we leave the EU, we face years of negotiations and neoliberal trade deals.”

In an email to his Newcastle East constituents, the MP Nick Brown said: “I am a moderate Remainer and have consistently believed that continued membership of the EU is in the national interest. I also believe it is in the overwhelming interest of the north-east of England. Therefore in such a confirmatory ballot, as in the 2016 referendum, I would campaign and vote to remain in the EU.”

At its conference, which opened in Bournemouth last night, the Liberal Democrat party will vote on whether to support revoking article 50 without a referendum if it forms the next government. In the latest Opinium/Observer poll, the Conservatives are up 2 points compared to a week ago on 37%, while Labour is unchanged on 24% and the Lib Dems are down one point on 16%. The Brexit party is unchanged on 13%.

A Labour party spokesperson said policy on Brexit would not be decided by the NEC and added: “Labour’s priority is to stop a disastrous no-deal Brexit. We can’t trust Boris Johnson to not crash us out of Europe on the worst possible terms.”