Jeremy Corbyn is under fresh pressure over the party's referendum stance as a string of Labour MEP candidates for the European elections have signed a pledge to campaign for a public vote on any Brexit deal.

The move comes ahead of a crucial meeting of Labour's governing body – the National Executive Committee (NEC) – on Tuesday, to sign off the party's European election manifesto.

Out of 70 Labour candidates for the election on 23 May, 22 have so far signed the pledge, committing them to campaign for a Final Say referendum on any Brexit deal, if elected next month, and vote to Remain.

The signatories include 12 current members of the European parliament, alongside Labour's leader in the parliament, Richard Corbett, and his deputy, Seb Dance.

The pledge – organised by the grassroots Remain Labour campaign – commits the candidate, if elected as an MEP, to campaign to give the people the Final Say, with a referendum that offers a choice between a Brexit deal and the option to Remain in the EU.

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Andrew Lewin, the founder of the Remain Labour group, said the overwhelming majority of Labour members wanted to stay in the EU, and he was delighted that 22 candidates had pledged to campaign for that outcome in the elections.

"Remain voters across the country who want to defeat Nigel Farage's Brexit party can now vote with confidence and enthusiasm for Labour candidates who have pledged to fight for our future, as members of the European Union," he said.

Jackie Jones, a Labour candidate for Wales, who has signed the pledge, said: "Now that we know more about what Brexit would mean for Wales, a confirmatory vote on any Brexit deal, with the option of staying as an EU member must be on the ballot paper."

The pledge also follows reports that the Labour leader is facing renewed pressure over the European elections manifesto from several leading unions.

According to Politics Home, the general secretaries of Unison, the GMB and Usdaw want Mr Corbyn to put a referendum pledge in the document to be signed off on Tuesday.

The shadow business secretary, Rebecca Long-Bailey, has hinted that Labour could sign up to a Brexit deal without a fresh public vote if the government makes significant concessions in the negotiations that entered a fifth week today.