He claimed on Sunday night that the decision was made because he had not received unambiguous assurances a Corbyn leadership would support Britain’s continued membership of the European Union in the coming referendum.
Umunna’s decision, which will be a blow to Corbyn, came after the newly elected deputy leader, Tom Watson, also insisted that Britain must remain a supporter of the EU as the world’s largest trading bloc.
The new Labour party leader is also meeting resistance about a plan to appoint John McDonnell, his campaign manager, as shadow chancellor.
Corbyn is trying to build a broad-based shadow cabinet in the wake of his massive mandate in the leadership election on Saturday. He spent most of Sunday at Westminster discussing his frontbench team with the reappointed shadow chief whip, Rosie Winterton.
Umunna spoke to Corbyn on Sunday afternoon and later issued a statement saying: “There are a number of key points of difference on policy which I believe it would be dishonest to deny exist. If Jeremy’s clear victory yesterday demonstrated anything, it was a desire for politicians to be true to what they believe – I want to abide by this.
“Jeremy should be free to appoint a shadow cabinet committed to implementing the policies on which he campaigned in the contest – I clearly had some differences in view on how we build a more equal, democratic, free and fair society.
“Given these differences, not least on the European referendum, I would find it difficult to abide by the collective responsibility that comes with serving in the shadow cabinet. 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; Jeremy has made it clear to me that he does not wholeheartedly share this view.”
Insisting he will not be a thorn in Corbyn’s side from the backbenches, Umunna said he would be working with Keith Vaz on an independent review into the support for Labour among ethnic minorities.
It also emerged that Shabana Mahmood, the shadow Treasury chief secretary, refused to work with Corbyn, while Ivan Lewis, the shadow Northern Ireland secretary, was sacked despite saying he was willing to stay on until Christmas to help tackle the political crisis in the province.
Watson has asked Corbyn to confirm his support for UK membership of both Nato and the EU and to allow a full debate within the party about a renewal of the UK’s independent nuclear deterrent. Watson is a strong supporter of Trident renewal, as are many leading unions, and said it was best to be honest about his position.
Watson admitted he was not sure of Corbyn’s policy on Nato, but Corbyn appeared to soften his position during his campaign for leadership –although he remained committed to rejecting another generation of British nuclear weapons.
Corbyn’s apparent refusal to rule out opposing UK continued membership in the EU reflects his desire to keep his options open and see what David Cameron negotiates with his EU partners. Many on the left fear that the prime minister wants to negotiate away hard-won employment rights or to ensure the City of London remains under-regulated. There is also a gathering hostility to the EU on the British left in the wake of the German treatment of Greece in its debt crisis.
Umunna’s statement is the clearest ideological difference to emerge between Corbyn and the MPs who opposed him. Many of those who are hostile to Corbyn know they have to tread cautiously since if they refuse to serve too readily, or without ideological justification, they risk being branded disloyal by a party membership that has just so overwhelmingly elected Corbyn.
Corbyn has been urged by some to appoint McDonnell to the shadow chancellorship, so his oldest political ally could lead his central political mission to build support for a strong anti-austerity economic programme.
But some leading Labour MPs said this would represent a declaration of war by Corbyn and run against his commitment to build a consensual shadow cabinet.
It is understood some senior figures are holding back from committing to join a Corbyn shadow cabinet until he clarifies that he will not give the shadow chancellorship to McDonnell, who is seen by some as too divisive a figure.
Corbyn has also been advised to appoint a woman to the post, such as Angela Eagle, or else the highly regarded shadow Welsh secretary Owen Smith.
The new leader has made a commitment to appoint women to half the shadow cabinetposts, and promotions are likely for Lucy Powell, Gloria Del Piero, Emily Thornberry and Lisa Nandy. Corbyn also met with defeated leadership rival Andy Burnham, but no agreement was reached about a possible role.
Sir Paul Kenny, the general secretary of the GMB union, called for MPs who opposed Corbyn’s election to leave the party if they planned to “snipe” and ponder their future in public.
Kenny said: “I understand why some people say they cannot serve in the shadow cabinet. I can respect that. But frankly, if they are going to spend the rest of whatever time it is constantly sniping, talking about rumours of leaving, then the best thing they can do is go. Go off into the oblivion [like] previous splitters from the Labour party have done.”
Len McCluskey, the Unite general secretary said: “I think you’ll see a shadow cabinet that’ll shock all of you. It’ll be a talented shadow cabinet, perhaps the best we’ve seen for many years. I think it’ll be balanced and it’ll be drawn from all elements of the party.
Watson told BBC1’s Andrew Marr programme that there was zero chance of a coup against Corbyn. “Labour party members will not accept that,” he said. “It’s fine for people to leave the front bench – I’ve done it myself a few times – but you have got to have the spirit of unity in your heart when you do it.
“And you want to try [to] make it work, but enjoy the freedom to speak out. I’m sure Jeremy will respect that. And what I do know from my own experience is there is always someone else that can do a front bench job.
“I understand the concern of my colleagues in parliament, this is a huge change for the Labour party [and] there is no point in denying this is a huge political realignment, too. But Jeremy Corbyn has got a huge mandate from our members.”
Former cabinet minister Lord Hattersley accused moderates refusing to serve of “self-indulgent nonsense” and an “abdication” of their duty to fight against extreme policies.
Writing in the Guardian, he said Corbyn was “incapable of leading the Labour party to victory at the next general election” and joined warnings of a return to the party’s 1980s electoral oblivion.
“Unless some drastic action is quickly taken, there is a real risk of disintegration,” he wrote. “Refusing to serve in the shadow cabinet is self-indulgent nonsense. As well as providing an excuse for Corbyn to promote nonentities, refusal denies senior MPs a ready-made platform from which to express their dissent.”
The party reported that a further 15,000 people had joined the party since Corbyn’s election on Saturday.
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