Maria Eagle’s new House of Commons office is appropriately located. Directly opposite lies the Ministry of Defence, the department she now shadows. “That’s the office we want,” she says, gesturing towards the neoclassical edifice.

In her first full-length interview since accepting the post, Eagle concedes that she was “a bit surprised” to receive the job. The former minister (she served for nine years in government and has been on the Labour front bench for 14) voted for Trident renewal in 2007 and remains a supporter of the UK’s nuclear weapons system, a stance antithetical to that of Jeremy Corbyn. “And that’s why I made sure I said to him, ‘Jeremy, you do realise that I believe in multilateral disarmament and that I’m pro-Trident, don’t you?’ ” Eagle tells me. “And he said ‘Yes, he did realise that’, still offered me the job, and I thought on that basis: ‘OK, he knows that, he’s not going to be surprised by my views.’ ”

Eagle, who was elected in 1997 to represent Liverpool Garston (now Garston and Hallwood), adds: “He wanted to know whether I was willing to undertake a review and I said ‘Yes’, obviously. We need to have a debate about these things. And on that basis he offered me the job and on that basis I took it.”

For Eagle, the debate over Trident renewal is one that Labour has avoided for too long. “I think there’s a mood out there for more transparency, not just in the Labour Party but in society as a whole. It’s not just in the Labour Party that the issue of defence spending is an issue. The Lib Dems had a big debate at their conference, the SNP obviously debate it all the time.

“I think at a time when you’ve got austerity and big cuts in public expenditure it’s reasonable for people to ask whether or not the money that we’re spending on defence generally and on a successor submarine, in particular, is properly spent.” The review, she pledges, will examine Trident “with a completely open mind” on “the basis of facts and figures”. Of the possibility that it could endorse unilateral disarmament, she says: “I’m not ruling it out.”

Eagle refuses to say whether Corbyn should grant a free vote on the issue but denounces the Conservatives as “despicable” for “seeking to play party politics” by potentially bringing forward the Commons decision to December. “For a start-off, they don’t have to have a vote on this, they’ve got a mandate. Since when did they decide it was necessary to have parliamentary votes on whether or not they should sign big contracts? I don’t remember them doing it about whether or not we should get the Chinese to build nuclear power stations in Somerset. I don’t remember them doing it on whether or not we should give Virgin the East Coast franchise.

“If they bring that vote forward, before Christmas, that is a despicable attempt to play politics with this issue and, quite disgracefully, with our national defence at a time of increased tension abroad . . . Their cover for it is that we don’t want it to be an issue in the Scottish elections. It’s going to be an issue in the Scottish elections whether there’s a vote before or not.”

She adds: “They’re here to exploit differences in the Labour Party. Well, we’re going to have a genuine debate about whether or not this is the right way for us to spend a significant amount of money over a significant number of years. We will have that debate properly. That kind of behaviour will be seen for what is.”

Labour conference delegates voted not to debate Trident at the party’s conference (“Delegates decided . . . you can’t say that it’s been kept off the agenda,” Eagle notes) but the issue dominated the final day after Corbyn told the Today programme that there were no circumstances in which he would press the nuclear button.

“For a potential prime minister to answer that question in the way he did isn’t helpful, it isn’t helpful,” Eagle says. “Now, I don’t have any problem with somebody who says, ‘Deterrence doesn’t work.’ But not everybody in the Labour Party believes that deterrence doesn’t work: I think it works. I think it certainly doesn’t work if you tell your potential enemies precisely what you are or aren’t going to do in given circumstances. I didn’t think it was necessary for him to answer that question.” Would she press the button? “I think the key thing for deterrence is not to tell your potential enemies what you’d do. It has to be an option, that is how deterrence works,” she replies.

David Cameron used his Conservative conference speech to denounce Corbyn memorably as “security-threatening, terrorist-sympathising, Britain-hating”. “It’s conference-speech rhetoric, isn’t it? That’s what it was, it was a personal attack,” says Eagle when I ask her to respond. “That’s what he does, he likes personal attacks.” She refuses to defend Corbyn’s statement that the assassination of Osama Bin Laden was “a tragedy”, which partly inspired Cameron’s attack.

“I’m not here to say that Jeremy was right or wrong about anything he’s ever said; you have to ask Jeremy to justify what he’s said about things. But I don’t think it helps to take comments out of context, is all I’d say, I don’t think it helps. Jeremy has said many things over the years from his perspectives. There’s no point asking me whether Jeremy’s saying the right things or not. He’s saying what he thinks and that’s fair enough.”

Of Corbyn’s own conference speech, which MPs criticised as addressed to the hall, rather than the country, Eagle says diplomatically: “He was only elected a couple of weeks before he had to make that speech. He made it in the way in which he wanted to make it and that’s fine. I’m not interested in criticising him about what he said.”

Eagle tells me that she supports the 2 per cent Nato defence spending target, which the Tories have pledged to meet. “I think it’s a good thing. As anybody who’s ever been a minister will tell you, one of the jobs of a cabinet minister is to fight your corner for your share of the resource.”

She goes on to charge the government with encouraging Russian expansionism through its defence cuts. “What’s happened is not unconnected: Russian spending going up in real terms, Nato spending going down. It’s not unconnected with, can’t be unconnected, with what we’re seeing the Russians do.”

Eagle and her sister, Angela, were the first twins to sit in the Commons together. After being linked with the post of shadow chancellor, Angela became shadow business secretary in the reshuffle and was hurriedly given the additional post of shadow first secretary of state after criticism of the absence of women from the four great offices of state (leader, chancellor, home secretary, foreign secretary) on the Labour benches. "I think my sister’s brilliant. Give her the biggest job going and she’ll do it and do it well," Eagle says when I ask her about the imbroglio. "Obviously it would have been easier if she’d been shadow chancellor: I’d have got a good settlement," she quips.

"Her experience and perspective on things has always been helpful to me, always. We’re very close and so that’s good, that’s good. We can be mutually supportive and we are. Without any concerns or worries about any downside. It’s pretty good being a twin, I’ve always thought so. I’d recommend it."

On the day we meet there is much unease among Labour MPs about the launch of the new Corbyn-aligned group Momentum, which they fear could be used as a vehicle for the deselection of critics. "It’s not clear to me precisely what it’s meant to be, so let’s see how things go," Eagle says in response. "I think anything that is used in that way isn’t helpful. We need to focus on the real enemy: that’s the Tories. We’re going to try and win the next election. We don’t do that by fighting each other, however that ends up happening."

And does she believe Labour can win the election? "I think it’s tough. But politics is changing, there’s a lot of churn going on, there’s a lot of things happening. We’re in a time of flux. There are opportunities. The Tories may fall apart over Europe. We don’t know what’s going to happen next week, next year. Let’s not write ourselves off. Let’s turn ourselves into a fighting force to sort them out. That’s what my constituents in Liverpool want to see us doing."