Jeremy Corbyn faced calls from members of his frontbench team to confirm Labour backs a referendum on a Brexit deal in its manifesto for the European elections.
Scores of Labour MPs and MEPs put their names to a letter calling on Labour's National Executive Committee (NEC) to commit to a public vote at a meeting on Tuesday to decide on the manifesto.
Labour leader Mr Corbyn declined to guarantee the commitment would be in the document, saying it would be for the NEC to decide.
Shadow Treasury ministers Clive Lewis and Anneliese Dodds and shadow minister for the disabled Marsha de Cordova are among the frontbenchers backing the call for a referendum.
The issue is incredibly divisive within the Labour hierarchy, with splits in the shadow cabinet over whether to campaign for another referendum amid fears it could torpedo the party's election chances in Brexit-voting former industrial heartlands.
Mr Corbyn, on the campaign trail in Leave-voting Peterborough, said Labour would be "prepared to consider" putting a Brexit deal to a public vote.
He said: "The national executive will decide on Tuesday what will be in the European election manifesto and we will reflect the decisions made (at) last year's Labour Party conference which were for a customs union, market access and rights protection within, with, the European Union.
"We would prefer to have a general election but failing that if we get that agreement we are prepared to consider putting it to a confirmatory vote. That is a decision the national executive of the party will make."
He added: "It's important that the party, which is a democratic party structure, makes those decisions. Sadly, or perhaps it's a good thing, I'm not a dictator of the Labour party."
After lengthy wrangling at Labour's conference in 2018, the party backed keeping "all options on the table, including campaigning for a public vote" if there was not a general election.
Richard Corbett, leader of Labour's MEPs and a member of the NEC, told BBC Radio 4's Today: "The problem we face now is that Brexit is turning out to be so different from what was promised three years ago.
"Remember they said it would be easy – it's turning out to be rather complex. They said it would save loads of money that would all go to the NHS – it's turning out to be costly.
"They said it would not damage the economy – we are seeing firms move abroad, jobs lost, especially in manufacturing.
"Because it's so different, it's right that it should go back to the people for a final sign-off."
Our letter to the @UKLabour NEC.
Our European elections manifesto must have a clear commitment to a confirmatory public vote on any Brexit deal. pic.twitter.com/MLUPOOERWZ
— Love Socialism Hate Brexit (@LoveSocialism) April 26, 2019
He dismissed the idea that Labour should "keep silent about our policy" of supporting a referendum to prevent a no-deal Brexit or the kind of agreement negotiated by Theresa May.
He said: "We have got to the point now where not holding another referendum is tantamount to saying to the general public 'you had your say three years ago, now you have to shut up and let the politicians serve up whatever they want to and you can't have a say on it'."
Pro-EU Labour MPs were angered by an election leaflet which contained no mention of a referendum, but Mr Corbett dismissed it as a "placeholder" pending a decision on the manifesto by the NEC.
"I expect the NEC to continue with what is Labour Party policy, which is to say that there should be a confirmatory public vote on any Brexit deal that this Government comes up with."
The letter from 75 MPs and 14 MEPs organised by the Love Socialism, Hate Brexit group sets out plans to beat Nigel Farage's Brexit Party with "a message of hope and solidarity", ahead of Tuesday's meeting.
"Labour has already, rightly, backed a confirmatory public vote," the letter states.
"The overwhelming majority of our members and voters support this, and it is the democratically established policy of the party.
"We need a message of hope and solidarity, and we need to campaign for it without caveats."
The group insisted the polls are clear that the May 23 Euro-election is a two-horse race between Labour and Nigel Farage's Brexit Party.
"To motivate our supporters, and to do the right thing by our members and our policy, a clear commitment to a confirmatory public vote on any Brexit deal must be part of our European election manifesto," the letter states.
"We understand the many different pressures and views within our movement, but without this clear commitment, we fear that our electoral coalition could fall apart."
Seamus Mallon, former deputy first minister of Northern Ireland and ex-Social Democratic and Labour Party MP, said Labour should "stop vacillating on the Brexit issue" and "give leadership"
It was "absolutely essential that another referendum is now held", he said in an interview with the Labour Party Irish Society.
"The issue of Brexit was put to the people, and the people spoke. But that, what they voted for then, bears no relation to what Brexit now is. And the whole circumstances of where Brexit now stands have changed very fundamentally since the initial question was put."