Jeremy Corbyn faces a showdown with his shadow cabinet over Brexit on Monday amid growing signs of a grassroots revolt if he fails to give full backing to a second referendum in which Labour would campaign to remain.

Several senior figures, including shadow foreign secretary Emily Thornberry, will demand a clear, full-throated commitment to another referendum, following the party’s disastrous showing in last month’s European elections, when it lost 11% of its share of the vote and half of its seats.

Thornberry said she would make it clear that it was time the party accepted that the next Tory leader was not going to offer a general election but would in all probability try to press for a no-deal Brexit – in which case Labour had to demand that the issue be put back to the people and campaign to stay in the EU.

Thornberry, who has already angered the Corbyn inner circle with her strong backing for a second public vote, said it was now clear that the election of the next Tory leader – probably Boris Johnson – was not going to lead to a general election.

Speaking on BBC Radio 4’s Today programme, she said: “So if that is right and given that we are likely to be in an impasse, I fear that whoever is leader of the Conservative party we are going to end up with no deal and we simply cannot have no deal without there being a referendum on it and with the option of Remain.”

Others, including deputy leader Tom Watson and shadow Brexit secretary Keir Starmer, are also expected to weigh in. The shadow chancellor John McDonnell is said by colleagues to be calling on Corbyn to make a keynote speech within the next few weeks to end the uncertainty.

While Corbyn said in the immediate aftermath of the European elections that the party would back a referendum on any Brexit deal, he has since sounded less sure, talking again of his preference for an election and saying a second public vote would be “some way off”.

Growing pressure is also coming from pro-EU Labour activists who are circulating anti-Brexit motions at constituency Labour parties (CLPs) for submission to the autumn annual conference in Brighton.

Emily Thornberry: ‘We simply cannot have no deal without there being a referendum on it.’ Photograph: Mark Thomas/Rex

By last night they said that more than 130 CLPs across the country had agreed to debate a motion which would commit Labour to “campaign energetically for a public vote and to remain”, and to “support revoking article 50 if necessary to prevent no deal.”

A similar motion will also be debated soon in Corbyn’s own Islington North CLP. It will note that the “vast majority of Labour members and voters oppose Brexit” and that conference “believes that Labour party policy must reflect this view and that we cannot go into any more elections without a clear policy for the UK to remain in the European Union.”

The campaign by Labour activists is similar to that mounted last year in favour of a second referendum. However this time the organisers say they have the clear backing of key unions including the GMB and Unison, and will not allow any motion that ends up at the party conference in Brighton in September to be fudged, as happened in Liverpool last year. The message from activists is that if Corbyn does not move soon, he will be forced to do so by party members.

The anti-Brexit MP Clive Lewis said: “Labour’s members are right, and I have no doubt that if the Brexit debate reaches conference floor, their views will win out. The result of a conference vote could only ever be an unequivocal shift towards a public vote and Remain. But we cannot allow things to get that far. It would be a disaster for the left if we spent an agonising, damaging summer waiting for September and only moved our position with a conference vote. We can shift our position here and now, win back millions of voters, and get on with the business of fighting for a radical Labour government.”

Ana Oppenheim, from the leftwing campaign group Another Europe is Possible, said: “The mood at the Labour grassroots is overwhelmingly in favour of an unequivocal push for a public vote and for Remain. A large chunk of the leadership has moved with the rank and file, with John McDonnell, Diane Abbott, Keir Starmer and Emily Thornberry all making it known that they now back a clear shift. After the European elections, we’ve seen a whole new layer of people come round to taking a clear stance.”

The organisation’s national organiser, Michael Chessum, added: “The hardening of the mood in the grassroots, combined with the fact that some of the big unions are backing a public vote, means that the balance of forces have moved in our favour, and we will not settle for a fudge this year. If we have to, we will take a full anti-Brexit position to conference floor, push it to a vote, and win.”