A shadow cabinet minister has broken ranks with official Labour Party policy to call for another Brexit referendum.
Shadow Northern Ireland Secretary Owen Smith said a vote should be held when talks with the EU have concluded.
In an article for the Guardian, he also called for Labour to back membership of the EU single market.
Labour's shadow Brexit Secretary Sir Keir Starmer said the party was "not calling for a referendum at this stage".
Jeremy Corbyn announced last month that Labour wanted the UK to be a permanent member of a customs union with the EU after Brexit.
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But Mr Smith, who unsuccessfully challenged Mr Corbyn for the Labour leadership in 2016, said that was not enough.
"If we insist on leaving the EU then there is realistically only one way to honour our obligations under the Good Friday Agreement and that is to remain members of both the customs union and the single market," he said in his article.
"I'm pleased my party has taken a big step in this direction by backing continued customs union membership, but we need to go further."
He argued that Labour "needs to do more than just back a soft Brexit or guarantee a soft border in Ireland".
"Given that it is increasingly obvious that the promises the Brexiters made to the voters – especially, but not only, their pledge of an additional £350m a week for the NHS – are never going to be honoured, we have the right to keep asking if Brexit remains the right choice for the country.
"And to ask, too, that the country has a vote on whether to accept the terms, and true costs of that choice, once they are clear. That is how Labour can properly serve our democracy and the interests of our people."
Conservative Brexit Minister Steve Baker accused Labour of "trying to frustrate the Brexit process".
"These comments show that on one of the biggest issues facing the country, Labour have no interest in making a success of it, betraying millions of Labour voters.
"Only the Conservatives are getting on with delivering what British people voted for, taking back control of our laws, borders and money."
Sir Keir Starmer said Labour's position was that the "benefits of the single market and customs union… need to be hard-wired into the final agreement".
"We want a strong relationship with the single market with no drop-offs in rights, standards or protections," he added.
Asked if Mr Smith was going against official party policy with his comments, Sir Keir said: "That's not a matter for me to comment on. My focus is on ensuring we are challenging the government appropriately on the negotiations with the EU. "
Mr Smith argued for a second EU referendum during his abortive Labour leadership bid – but Jeremy Corbyn has consistently ruled one out, saying that the public had come to a decision, and has insisted the UK cannot remain in the single market.
Other shadow cabinet members have appeared more open to the idea of another referendum.
In January 2018, shadow foreign secretary Emily Thornberry told the BBC's Andrew Marr Show she would consider it only in exceptional circumstances – "if 90% of the population was now saying we will stay in the European Union".
Labour's deputy leader, Tom Watson, did not rule out a second referendum, in a December 2017 interview with BBC Radio 5 Live's Pienaar's Politics, but added: "What we actually want is a negotiated settlement."
Anti-Brexit Labour backbenchers such as Chuka Umunna, a member of the cross-party Open Britain campaign, have also argued for a second referendum.
Mr Umunna said Mr Smith was "absolutely right" to say that Labour "needs to go further in opposing the government on Brexit", saying the Leave campaign had made promises "we now know cannot be delivered on".
"If we are not going to be offered the Brexit we were promised we have every right to ask whether Brexit is the right path for the country," he said.