Shadow Secretary of State Hilary Benn has rowed back on a pledge by his predecessor to set out the conditions for a border poll.
The Labour MP, widely expected to be appointed secretary of state after the next election, told The Irish News he was "not persuaded" that a formula can be produced that would trigger a referendum on constitutional change.
Mr Benn also echoed Keir Starmer's view that a border poll is "not even on the horizon".
Last year, his predecessor Peter Kyle said he would set out the criteria for calling a referendum on Irish unity if Labour were in power.
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Taoiseach Leo Varadkar has been among those calling for clarity around the conditions for a border poll, while in a survey published last month 55% of people said the basis under which a referendum on the north's constitutional future will take place should be clearly spelled out.
Under the Good Friday Agreement, the secretary of state must call a referendum if it appears likely a majority of those voting would want the region to leave the UK.
However, the agreement fails to specify the criteria.
In 2020, victims campaigner Raymond McCord failed in his legal bid to compel the British government to put in place a policy that made clear the circumstances for a border poll.
Speaking exclusively to The Irish News in Dundalk on Thursday, Mr Benn said he had examined the court's judgment and "read carefully" the Good Friday Agreement.
"I must say I am not persuaded that you can produce some formula that says 'six opinion polls over a period of whatever'," he said.
"In the end, it's a political judgment and the Good Friday Agreement was written with that aim in mind – it says very clearly that the secretary of state, whoever she or he is at that moment, once they come to believe that it's likely that a poll would go in favor of Irish unity, then you must hold one."
The shadow secretary of state said the conditions were included in the accord and that he intends "to cleave very closely to the letter and the spirit of the Good Friday Agreement".
He said it was up to those advocating Irish unity to set out their proposition.
Ireland's Future CEO Gerry Carlile said the failure by the current or future British government to set out the criteria "fits well with the ongoing contempt that London is showing for the people of the north from all backgrounds".
"Our view is that British government has abandoned the people here," he said.
"The sooner the Irish government plan for a border poll and approach the British government and other stakeholders with that plan, the better."
