Jeremy Corbyn has sacked Labour's shadow Northern Ireland secretary Owen Smith because he called for a second EU referendum.
Mr Smith, himself a one-time challenger to Mr Corbyn's leadership, was asked to "stand down" for breaking ranks with official party policy.
He has been replaced by Tony Lloyd, who Mr Corbyn said "is a highly experienced former Government minister who is committed to ensuring that peace in Northern Ireland is maintained and helping to steer the devolution deal back on track."
Mr Smith said he had been sacked for his "long held views" on the ramifications of Brexit.
He wrote on Twitter: "Just been sacked by [Jeremy Corbyn] for my long held views on the damage Brexit will do to the Good Friday Agreement [and] the economy of the entire UK.
"Those views are shared by Labour members [and] supporters and I will continue to speak up for them, and in the interest of our country."
Tensions between the two appeared to have settled since the former's appointment to the shadow cabinet in 2017.
Mr Smith stood as the only opposition candidate to Mr Corbyn's leadership in the 2016 Labour leadership election. Mr Smith was defeated with 38.2 per cent of the vote to Mr Corbyn's 61.8 per cent.
Colum Eastwood, leader of the Social Democratic and Labour Party in Northern Ireland, wrote on Twitter: "Very sorry to hear that we're losing [Owen Smith] as shadow secretary of state.
"Owen has a great grasp of the issues affecting us here. He particularly understood the challenges of Brexit and was very supportive of sensible solutions to held us avoid a hard border."
Outspoken Labour MP and Remainer Chuka Umunna sharply criticised the decision, saying it was "extraordinary" that a shadow cabinet member "doing an excellent job" should be sacked.
He said: "Sacking [Owen Smith] does not alter the truth of what he says: staying in the Customs Unions *and* the Single Market is the only way to avoid a hard border on the island of Ireland.
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"It's extraordinary that a shadow cabinet member - doing an excellent job in their brief - should be sacked for standing up for our principles and advocating a Brexit policy that commands the overwhelming support of our members, supporters and voters.
"What has happened to our party?"
While Sir Vince Cable, leader of the Liberal Democrats, wrote: "Shame to see [Owen Smith] sacked for simply repeating the view of the majority of Labour members and, I suspect, MPs."
Shadow home secretary Diane Abbott said: "Owen Smith was a valued colleague, he wanted to make a contribution to the debate and he will be able to continue to do so.
"What he can't do is sit on our front bench and advance a position which is simply not Labour Party policy."
Ms Abbott herself wrote to constituents in November saying she would "argue for the right of the electorate to vote" on the Brexit deal.
But she told BBC Radio 4's Any Questions: "We have debated this in the Labour Party, debated it seriously because we take the referendum result seriously.
"But we are not calling for a second referendum."
The feud comes as a number of Labour MPs criticised Mr Corbyn's role in a scandal over the removal of "anti-Semitic" mural in London.
Mr Corbyn suggested artist Mear One was "in good company" because the Rockefeller family had covered over a mural featuring Lenin in their New York development.
It is understood that at the time of the post in 2012, Mr Corbyn did not condone the artwork but was expressing concerns about free speech.
The Standard approached Mr Corbyn and Mr Smith for further comment.
