Jeremy Corbyn has handed closest leadership rival Andy Burnham a key shadow cabinet role as he seeks to bolster party unity.
The newly-elected Labour leader appointed Mr Burnham to the shadow home secretary role vacated by Yvette Cooper.
Ms Cooper, who came third in the race to succeed Ed Miliband and made clear during the campaign she would not serve on the veteran left-winger's frontbench, will head a taskforce examining the refugee crisis.
After a day of behind-the-scenes negotiations, the first confirmed appointments also included Hilary Benn as shadow foreign secretary, Heidi Alexander as shadow health secretary and Lord Falconer, a close ally of Tony Blair, as shadow justice secretary.
But Chuka Umunna and Mary Creagh added their names to a lengthy list of senior figures returning to the backbenches over stark differences with Mr Corbyn over key policy areas.
Mr Umunna, who was shadow business secretary and briefly a leadership candidate, said he "clearly had some differences in view" with Mr Corbyn, most significantly over Europe.
He expressed particular concern that Mr Corbyn appeared unwilling to guarantee that Labour would campaign to remain in the European Union in the forthcoming referendum on continued membership.
"It is my view that we should support the UK remaining a member of the EU, notwithstanding the outcome of any renegotiation by the Prime Minister, and I cannot envisage any circumstances where I would be campaigning alongside those who would argue for us to leave," he said.
"Jeremy has made it clear to me that he does not wholeheartedly share this view."
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.
Mary Creagh, the shadow international development secretary, said she had "decided to return to backbenches and campaign on Labour 'yes' in the EU referendum, Syria refugee crisis, and a better deal for people in Wakefield".