Sinn Fein has emphatically rebuffed an SDLP call to end its abstentionist policy and take up its Westminster seats to fight the impact of EU withdrawal.
SDLP leader Colum Eastwood predicted Labour's major policy shift backing a longer post-Brexit transition period had the potential to prevent a 'hard border' scenario.
After the loss of the SDLP's three MPs in the general election, Westminster has been left without a nationalist voice given Sinn Fein's refusal to take the oath to the Queen.
Mr Eastwood called on Gerry Adams to put Irish national interest ahead of party pride.
"Up to this point Sinn Fein and Gerry Adams have used the excuse that their votes would make no difference because of the Brexit position of the Labour Party. Now that the Labour Party's position has changed, Sinn Fein's should too," he said.
"They should reflect on the fact that their continued abstentionism is only serving themselves instead of the broader Irish national interest.
"With the continued absence of an Executive in Belfast, and with the DUP holding the Tory party over a barrel, we must do everything in our power in Ireland to fight against the establishment of a new border. Party pride should not get in the way of that fight."
The SDLP threw down the gauntlet after Labour argued a longer transition period following the Brexit date of March 2019 would give Northern Ireland more time to resolve the border checks problem.
"I welcome the significant shift in the British Labour Party's Brexit position. It is a position which has the potential to change things on the island of Ireland," Mr Eastwood added.
"Votes on continued membership of the single market and the customs union at Westminster have just become very winnable. That would mean preventing any hardening of the border in Ireland."
Mr Eastwood cited the Conservatives' failure to win an outright majority in June's election, saying only a few Government defections would be needed to win votes on Brexit. He said it was a scenario that was "increasingly likely as the value of sterling continues to plunge and economic forecasts continue to look gloomy".
However, Sinn Fein MP Chris Hazzard said there was no prospect of the party ending its policy of boycotting the House of Commons.
Mr Hazzard, who won the South Down seat from the SDLP's Margaret Ritchie, is one of seven Sinn Fein MPs.
"Election pledges may not mean much to the SDLP but they do to Sinn Fein," he said.
"Sinn Fein stood on an active abstentionist ticket in the Westminster election and were overwhelmingly endorsed by the nationalist electorate.
"The SDLP lost all its seats after failing to produce a single example of substance to sustain their claim of making a difference by sitting in the British Parliament.
"They are now asking Sinn Fein to pick up their failures where they left off."