Sir Keir Starmer has said that Britain's withdrawal from the European Union "can be stopped".
Labour's shadow Brexit secretary told Sky News: "Brexit can be stopped. But the real question is: what are the decisions we are going to face over the next few weeks and months?".
His comments contradict those made by party leader Jeremy Corbyn, who told a German magazine last week that "we can't stop Brexit".
However, Sir Keir insisted the Labour leadership was fully signed up to the position agreed at the party's annual conference in Liverpool - including the option of another "public vote" on Brexit.
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He added that there were some "slightly different voices" within the party on the issue, but said that "everybody has coalesced" around a common position.
Last November, Sir Keir claimed there was "no dispute that Britain will leave the European Union in March 2019", but in an interview with talkRADIO's Ross Kempsell at the Labour party conference in September, refused to rule out extending Article 50.
Sir Keir said: "We don't know when we're going to get a deal. The October deadline might slip to November, November might slip to December.
"I don't think at this stage anybody's talking about extending Article 50, but if it has to be extended, quite frankly, it'll be because of the collapsing failure of the discussions and the negotiations."