The scale of the deep divisions at the top of the Labour Party have been laid bare as shadow cabinet ministers put themselves on a collision course with the new leader over a series of crucial issues.
Jeremy Corbyn, an anti-war campaigner, opposes the replacement of the nuclear weapons system, has questioned the role of the Nato and is expected to oppose military intervention in Syria.
But senior party figures have set out policy red lines that put them at odds with Ed Miliband's successor and described his victory as an "earthquake".
Lord Falconer, a close friend of Tony Blair who served in the former prime minister's government, said he had "no idea" if Mr Corbyn would be prime minister following the 2020 general election.
The shadow justice secretary told BBC One's Sunday Politics: "What's happened is an earthquake in the Labour Party."
Lord Falconer set out his disagreements with Mr Corbyn's views on a wide range of policy areas, including defence, foreign affairs, welfare, education and the economy.
"He said during the course of the leadership election that, for example, he might think about withdrawing from Nato. That would obviously be something I could not agree with," he said.
Lord Falconer said he was in favour of renewing Trident but said it was for the party to debate the issue.
The peer also set his face against suggestions by Mr Corbyn and shadow chancellor John McDonnell that the policy that gave the Bank of England independence could be reversed
"I'm in favour of keeping the Bank of England independent," the peer said as he also set out his opposition to any plans for nationalising the banks.
The former Cabinet minister said he did "not want to see" the schools academy programme abandoned and described the welfare cap Mr Corbyn is fiercely opposed to as "a quite sensible measure".
He also dismissed the leader's suggestions that the gas and electricity national grids should be nationalised.
"I'm not in favour of that. I think that would be going too far. I think that would be a vast expenditure when there are so many other things we should be spending money on so that would not be the right thing to do."
Asked if the shadow cabinet faced some robust discussions, he replied: "Yes, I think you are right."
Lord Falconer said the new leader was "a very pluralistic person" who was "very open to debate".
The peer said he had talked to Mr Blair about accepting a position in the new frontbench team.
"It's a private conversation, I'm not going to say what he said, but we remain close friends," he said.
Mr McDonnell's calls for the former prime minister to be tried for war crimes over Iraq are "complete nonsense", he told the programme.
"Tony Blair is not a war criminal. That's just off the page."
Asked about the possibility of Mr Corbyn apologising on behalf of the party for the Iraq war, he replied: "I don't think we should apologise but that's a matter for Mr Corbyn to decide, if he's going to apologise."
Senior Labour figures have privately pledged support for air strikes in Syria with half of Mr Corbyn's shadow cabinet ready to back intervention, according to The Sunday Times.
Lord Falconer said he believed the Opposition should back action if a proper case could be made.
"My view in relation to the attacks on Syria is that if there is a military and legal basis for doing then we should do it," he said.
Lord Falconer's decision to disclose his policy red lines come after shadow foreign secretary Hilary Benn set out his support for Trident and Nato.
Mr Benn said military alliance was the "conerstone of our security" and insisted he did not see Britain withdrawing.
He told BBC One's Andrew Marr Show: "My view is that we need to maintain an independent nuclear deterrent."
He added: "We live in a differently dangerous world now. We need a continuous at sea deterrent. We need to do it in the most cost effective way and that is the view which the Labour Party, including the Labour Party conference, has taken for many years."
Asked if he could serve in the shadow cabinet if the party voted to leave Nato and get rid of Trident, he replied: "I don't think that is going to happen."
Labour former home secretary Charles Clarke suggested the party was preparing to fight the next election without Mr Corbyn.
He told Murnaghan on Sky News: "I think it is a question of whether he will get so far as the 2020 general election but I think the party will be very keen to give him a chance and be working, of course, on its own proposals to try and ensure that we can contest the Tories effectively at the next election.
"But, at the end of the day, it is very much in Jeremy Corbyn's own hands as to how long he lasts."
"It's obviously been a very bad week for him," he added.
Mr Clarke said there was "absolutely no chance of defections" and claimed Lib Dem leader Tim Farron, who has suggested he has spoken to more than two Labour MPs about jumping ship, was making "mischief".
Asked if members were giving the new leader enough rope to hang himself, he replied: "We'll have to see if he does hang himself in any way. So far the signs for Jeremy Corbyn aren't particularly good."