Jeremy Corbyn has begun a shake-up of the shadow cabinet with shadow Northern Ireland secretary Ivan Lewis the first confirmed casualty.
With the newly-elected Labour leader holed up in the Commons engaged in the thorny jigsaw of drawing up a new team, Mr Lewis said his offer to stay on to help respond to the ongoing political crisis at Stormont had been rejected.
Mr Corbyn was declared Ed Miliband's successor on Saturday on the back of a surge of support from activists that saw him attract a massive 59.5% of votes - topping the ballot among party members as well as trade unionists and new supporters.
But the immediate resignations of a string of senior figures who declared themselves unable to serve in his top team underlined the task he faced forming a shadow cabinet that balanced socialist allies and respected moderates - while meeting an aim for a 50:50 gender balance.
He faced warnings from several fronts today - including deputy leader Tom Watson and Unite union boss Len McCluskey - that he would have to temper radical policies such as opposition to the renewal of the UK's Trident nuclear deterrent in the interests of party unity.
Mr Lewis, who backed Blairite Liz Kendall for the leadership and during the campaign criticised Mr Corbyn's association with extremists "who have engaged in anti-Semitic rhetoric", had stressed the need for the party to play an "equidistant role" in the power-sharing crisis.
Mr Corbyn controversially invited two former IRA prisoners to speak at Westminster two weeks after the Brighton bomb in 1984 and has been accused by some unionists of being too close to Sinn Fein.
"Earlier today I offered to remain as Shadow Sec of State for NI for the time being in the light of the current political crisis," Mr Lewis wrote on Twitter.
"I thought it was the right thing to do. Jeremy has decided to offer the role to someone else. I wish my successor well at this crucial time."
Before being informed of the decision he had written that the crisis in the power-sharing administration in Northern Ireland was "the most serious threat to peace and stability for many years (and) should be a priority for the new Labour leadership.
"Labour has a proud legacy and therefore special responsibility. It is essential we continue to work with the Government on a bi-partisan basis and maintain an honest broker, equidistant role between Northern Ireland's political parties and the loyalist and nationalist communities."
In a signal of the likely battles to come, Mr Watson - seen as a crucial linchpin in securing party unity - conceded that several areas of conflict between Mr Corbyn's long-held views and party policy "have got to be worked out".
Left-wing ally Diane Abbott suggested that he would not seek an exit from the EU or Nato but would not drop his opposition to Trident.
Senior trade unionists, while warmly welcoming Mr Corbyn's election, insisted compromise would be essential.
Unite general secretary Len McCluskey said Mr Corbyn no longer had the "luxury" of expressing views when he was a backbench MP.
"Once you become the leader of an organisation, it is the policies you are honour-bound to deliver," he told the Press Association, noting that he had to drop his own unilateralist view when he became the union's boss.
Prime Minister David Cameron, who last night telephoned his new opponent to congratulate him, issued a stark warning that Labour under him posed "a threat to our national security, our economic security and your family's security".
As Mr Corbyn deliberated his options, shadow chief secretary to the Treasury Shabana Mahmood became the latest to rule out serving under him.
Defeated leadership rivals Yvette Cooper and Liz Kendall are among those to have declared they would not take a role - with Tristram Hunt, Rachel Reeves, Chris Leslie, Emma Reynolds and Jamie Reed also returning to the backbenches.
It remained unclear whether Andy Burnham - who came a distant second in the leadership contest and had said he could work with Mr Corbyn - and Chuka Umunna, who issued a plea for unity, would accept jobs.
The only confirmed appointment so far is Rosie Winterton, who stays as chief whip.