Aged 17, broke and living alone, Emily Thornberry had a number of low-paid jobs. She was a barmaid at the Hammersmith Palais; there was also a stint in a factory stacking boxes and folding cards. Neither of these youthful experiences, however, has proved quite as indelible as the time she spent as a cleaner on the Townsend Thoresen ferry from Dover to Zeebrugge. The crossing was often rocky and as, a result, passengers were frequently sick. On one occasion, she arrived dutifully at the loos, mop and bucket in hand, only to find every last basin and lavatory pebble-dashed with vomit. What did she do? Was she tempted to make a sudden break for the upper deck and fill her lungs with the North Sea breeze? No, not a bit of it. “I quietly locked the door behind me and I just got on with it,” she says, wrinkling her nose.

It’s hard to resist making a metaphor out of this anecdote, given that many people are wondering for how much longer the vast majority of Labour MPs intend to put up with the stench that currently rises from their party. Our meeting takes place in Thornberry’s constituency office in Islington a week after the screening of the Panorama programme in which former Labour staffers alleged that key Labour figures had interfered with investigations into complaints of antisemitism in the party: a period of days during which, to put it mildly, quite a lot has happened. A group of MPs, among them Yvette Cooper and Stephen Kinnock, has urged the party’s national executive committee to set up an independent investigation into the allegations. Some 200 former and current Labour party staff, members and supporters have challenged Corbyn to resign if he cannot renew trust in its dealings with its employees (the whistleblowers had to break nondisclosure agreements in order to speak out). Sixty Labour peers – a full third of the party’s members in the Lords – have taken out an advert accusing Corbyn of having “failed the test of leadership”. The atmosphere grows more febrile by the minute.

And yet, at the top, the denial goes on; pegs are still on noses. On social media, elements of the party have attacked both the BBC and the whistleblowers, rather than focusing on the allegations themselves. Jeremy Corbyn has spoken of the programme’s “many, many inaccuracies”. Labour has demanded that it be removed from the iPlayer (a request the BBC has refused). Len McCluskey, the leader of the Unite union, has called Tom Watson, the deputy leader of Labour, “a fucking disgrace” for having dared to criticise Jennie Formby, Labour’s general secretary, while she is undergoing chemotherapy.

There is a feeling that something has to give, but it’s hard to see what exactly this might be. For while an MP such as Thornberry, who is shadow foreign secretary, talks of the “distress” the programme caused her, she will not countenance my use of the c-word – complicity – to describe her ongoing support for Corbyn, whom she insists is “very distressed” by the allegations. Though some believe that the buck for all this stops with him and that he needs to go, she is adamant – at this point, feel free to picture a can of air freshener – that not only will “good, decent” Corbyn lead the party into the next election, but that he can, and will, win it. “We’re the MPs for Islington,” she says when I ask about her relationship with the leader, an old photograph of whom can be seen on the half-empty pinboard behind her (his hair still long and brown, he brings to mind swinging Howard Kirk in Malcolm Bradbury’s campus novel The History Man). “We’re close.” Doesn’t she feel let down by him? “I think that…” For a second, she hesitates. “He took too long to make some important decisions. I felt disappointed that we took as long as we did to accept the international definition of antisemitism [Labour adopted the International Holocaust Remembrance Alliance definition in full in 2018]. But that was because he found the issue of antisemitism distressing; he was accused of it and he just isn’t antisemitic in my view. I do know how very hard this is for him.”

Thornberry with Jeremy Corbyn: ‘We’re the MPs for Islington. We’re close.’ Photograph: Darren Staples/Reuters

Thornberry watched Panorama at home with her husband and daughter. The personal testimonies of the young Labour staffers, who spoke of the effect of their treatment by party officials on their mental health, upset her deeply. But she also felt that the programme did not accurately reflect recent changes to processes that have been made. Nevertheless, as she notes quietly: “It goes on and on…”

What, then, is she going to do about it, apart, that is, from taking the trouble to appear on The Andrew Marr Show? (She said that Labour must allow the Equality and Human Rights Commission, currently investigating the party over allegations of antisemitism, free rein to reform its mechanisms for dealing with complaints.) Isn’t there a sense that, for some on the left, the allegations are regarded as hysteria, as simply a weapon with which to attack its leader? And that while these attitudes go unchallenged by that leader and his acolytes, nothing will change? But she doesn’t agree. “I don’t think that’s fair,” she says. “I think some of them have been slower to understand how serious it is. I think it’s right that there were people who were dragging their feet initially and who hid behind the fact that antisemitism in Labour may be no greater than it is in the Conservatives or the rest of society. But while I’m ashamed that the EHRC is doing an investigation, the only piece of hope is that maybe together we can work out a new system, a robust system, an example of good practice.”

Beyond this she will not go. Do not expect the drama of resignation or departure from her. “I am very lawyerly about this,” she says (Thornberry was previously a barrister). “We either have politicians interfering or we don’t. If we are going to have a quasi-judicial process, which I think is the right thing to do, that doesn’t sit with politicians being involved. I am torn about it. I really do want to get involved, but we also have a structural thing, which is that the NEC is in charge of the Labour party, and I’m not on the NEC. What I’ve tried to do throughout this period is to keep connections with the Jewish community – I go to everything I’m invited to – and to try and steer a line on our foreign policy that is reasonable, robust and even-handed and that is very difficult with issues like the Middle East, which is quite often a spur for a lot of this [antisemitism on the left].”

Is Corbyn badly advised? Are those who surround him the problem? “I don’t want to go into that. I can speak on the record and advisers can’t. It’s not right for me to say things against them and for them not to be able to answer back.” How would she describe her relationship with Seumas Milne, the party’s director of strategy? (Milne is one of those accused of having interfered in the antisemitism cases.) “I don’t know. I think it’s fine. Seumas is very charming. I’m sure that anyone who watches closely can see that we disagree, but these things are done behind closed doors and it wouldn’t be right for me to comment on them.” What about Tom Watson, increasingly critical of Corbyn and those around him? “I don’t really know Tom.” On Marr, she, like McCluskey, insisted that Watson was wrong to criticise Jennie Formby. Didn’t she think it was a bit cheap of McCluskey to deploy Formby’s cancer in the way that he did? (Margaret Hodge, a Jewish MP, has said that her illness has been “weaponised”.) “I think she’s… a really extraordinary woman. She’s having chemotherapy, but nevertheless she is doing what she can and she went to shadow cabinet last week; she took a break from her chemo and gave a presentation on antisemitism and what was happening. I asked her some questions. She looked glorious, but she had no hair, and we know how ill she is, and from what I can remember Tom didn’t ask her any questions – and then he writes a public letter having a go at her. On a human level, I just don’t think it’s right.” Perhaps it’s worth noting here that McCluskey, who has a child with Formby and who is also bound to play an important role in any future leadership contest, is said to be a fan of Thornberry’s.

On the road with Labour Remain battle bus in Birmingham, June 2016, with fellow MPs. Photograph: Christopher Furlong/Getty Images

Is she on manoeuvres? She insists that she isn’t. “I’m not plotting. You can ask all parliament. I want a Labour government and I want to be foreign secretary in that government and I think I will be bloody good at it.” But I’m not sure. It’s interesting how visible she has made herself lately. A jolly interview about her children’s pet rats in the House of Commons magazine one day, an appearance on Marr the next. Some will observe that she is well placed to appeal to a wide cross‑section of the party; she supported Yvette Cooper as a candidate for the leadership in 2015, but ever since has served under Corbyn, remaining, through thick and thin, supportive of him. There’s something straightforward about the way that she tells me, just before I leave, that it’s “about time” the Labour party had a female leader.

Thornberry debating the leak of Kim Darroch’s emails in parliament. Photograph: Universal News And Sport (Europe)

Above all, there is her change of heart on Brexit. Having supported Remain during the referendum campaign, once Britain had voted to leave, she was firm. Though she insists she can’t remember saying this now, she accused pro-EU Labour MPs of being part of an “increasing normalisation of dishonesty”. She had, she said, no time for rebel MPs who “prance” in the media, demanding opposition to Brexit. But then, on the night of the European elections, in which Labour did so badly, and the Liberal Democrats did so well – in Islington, they beat Labour – there she was on the television, claiming that Labour should have said that any deal offered on Brexit by the current government should be put to a confirmatory referendum, that Remain should be on the ballot paper and that Labour should campaign to stay in the EU. (If she received a telling-off for this from the leadership, it hardly matters now, for this is the position since adopted by the party.)

“Yeah, well,” she says. “This is how it goes. I campaigned wholeheartedly to remain, but I thought we were going to lose. I went on a bus around the country and we would get Labour MPs coming on it in order to campaign in other people’s constituencies because they didn’t want to campaign in their own; they didn’t want to upset their constituents. I thought, ‘Oh, God. People don’t realise how bad this is’, and we did lose and I was very upset. However, we live in a democracy and I have to do what I am told.” Her hope was that we would leave, but not “go too far”, a means of respecting the votes of the 48% as well as the 52%. But the government’s negotiation never passed Labour’s so-called tests and now here we are, facing “no deal on one extreme and no Brexit on the other”. For this reason, she thinks Labour should be building the barricades for remaining. “We need to be truthful [to Labour Leave voters]. We need to tell people that what the Tories are trying to do is different to what they were promised and that we want to check back with them and have another referendum. We need to say that we have run out of other options and that we are sorry.” Is it nice to wake up in the morning and know that she can speak out in favour of Remain again? You can almost see her shoulders drop. “Of course. Because it’s in accordance with what I want anyway.”

What does she think is going to happen in the next few months? “If anyone says they know, they’re lying. But we should prepare for an election in the autumn. I don’t think he [Boris Johnson] can leave with no deal without either a referendum or a general election. The consequences for the country are so dreadful they will never be forgiven. You can’t do something so extreme as a minority party, and with a leader who has been elected largely by elderly people from the home counties of limited background and experience of life, without going back to the people. He might go for an election if he is way ahead in the polls, but he’s so unpredictable; he might also have the chutzpah to ask the people to choose between no deal and Remain.” How would that go? “It’s unpredictable. But I learned this weekend that no one who was born in the 21st century voted in the referendum – isn’t that amazing? – and it’s all about them and their future and they have a clear idea of what they want. I would say a narrow win for Remain, but it can’t be guaranteed.”

On television, Thornberry can come over as patronising. What works at the dispatch box – masterful when covering for Corbyn at prime minister’s questions; she once witheringly described the cabinet’s handling of Brexit as “Reservoir Dogs remade by the Chuckle Brothers” – doesn’t always seem quite right away from the bear pit. But in person, she is as warm as any politician I’ve ever met, and pretty human too. There is something ebullient about her, an energy that goes undimmed in spite of the political madness that is all around. Am I right about this? She seems – to be blunt – to be far less depressed about everything than me and I can’t work out why this should be. “Yes,” she says, quietly. After this, she is silent for longer than at any time in our conversation, until she finally adds: “I have had some pretty hard times in my life, and I have got through them, and it is important to remember that: you are stronger than you think. Keep going. I feel people’s hopes are pinned on me. I have got to keep going. But I am fairly robust.”

Thornberry with her husband, Christopher Nugee. Photograph: Andrew Parsons / i-Images

She credits some of this buoyancy to her husband, Christopher Nugee, a high court judge. They met when she was 22 and still at bar school (she’d attended not knowing what else to do and because she was able to get a grant for it). She grins. “Yes, an A1 husband. I was very angry then: chippy. I was finding it difficult to find my way. I was deeply lacking in self-confidence.” She used to play bridge with him and he was a generous partner. When she put down her terrible hand, he would compliment her on it nevertheless and then go on to win anyway. “He was clever and kind… and his big blue eyes.” They married when she was 30. “It took me a long time to be persuaded that he wasn’t going to leave me. But he just loved me and he has continued to do so. He has given me huge stability and support.”

She and the public school and Oxford-educated Nugee were from wildly different backgrounds, though not all of this comes down to social class. It’s more complicated than that. Thornberry, who was born in Guildford in 1960, is from a solidly middle-class background: her father, later assistant secretary general of the United Nations, was a Cambridge-educated barrister; her mother was a teacher and a Labour councillor. But her early life is imprinted with a dramatic “before” and “after”: when she was seven, her father left her mother for another woman and the family’s situation changed radically. Her mother moved into a council house and Thornberry and her siblings had free school meals; early on, they were so broke that – according to legend – their cats were put down. “It was a different time,” she says. “It [divorce] didn’t happen then.” It wasn’t shameful, exactly. But it was unusual enough for comment and her mother felt it. She would sit up late, telling Emily how terrible her father was.

Is is true about the cats? “Yes, it is true,” she says. That suggests a catastrophe, doesn’t it, and of a kind that would form a person utterly? “Yes,” she says, again. “And there was a double whammy. When I was 15, I had a row with my mum, and I went to live with my dad in Hammersmith, and one day, he went to New York and he never came back [he’d had a job offer from the UN]. Suddenly, I was on my own in London. When you’re 17, you live with it, but when I was the mother of a 17-year-old myself, I became furious about it. How irresponsible.” Naturally, she couldn’t go back to her mother. “I couldn’t admit to her what was going on. My pride was at stake.” Was she lonely? “I was terribly lonely.” Did her father feel guilty? “I don’t know. He never told me that he felt guilty.”

Their father’s abandonment affected his children in different ways. “He had six children. I have two full brothers, a half-brother who died and two half-sisters. They all had strong criticisms of him. Dad came back, having been away for decades, with dementia, and had to be looked after. My brother found a care home, but having done that, he said: right, that’s it. I was the one who had to go and see him. I saw him disintegrate. He wasn’t someone who lived on other people’s emotions. He was self-contained and never complained; his wives and girlfriends would tell me what was going on in his life. But it was terrible to see. He would talk about it. He would say he felt he was drowning in the sea, the waves coming over him, that he was disappearing. I had to go and see him and to begin with, I really resented it – that he had disappeared with so little involvement in our lives and now he expected us to look after him. But as he disintegrated in front of me, I just felt great pity for him and great love for him. I thought: you’re an old bastard, but I do love you.

“When he died, I was really sad, whereas my brothers turned up at the funeral raging. They still felt the same anger. Some of my siblings, if they were honest, would say they still suffer from the way he treated them. But I am all right. He was a great man, but he was terribly flawed as a father.” The awareness she’d had during his illness that visiting him would save her in the end – that this was important emotional work – turned out to be right. “It helped me. It absolutely helped me. That was the point. My attitude to his death was calm, peaceful, balanced.” Her voice is very low now, but there is no self-pity in it or even much sadness. All this has made her what she is – it’s a part of her – and she knows it.

Canvassing in 2005, the year she was first elected to parliament, following an unsuccessful 2001 campaign in Canterbury. Photograph: Bridget Jones

Thornberry failed her 11-plus and was bullied at her secondary modern. Her teachers had no hopes for her. But, alone in London, working as a cleaner and a barmaid, she resat her O-levels, took her A-levels and won a place to read law at the University of Kent. After bar school, she joined Michael Mansfield’s chambers. She was the unsuccessful Labour candidate for Canterbury in 2001 and was elected to parliament in 2005, replacing Chris Smith as the MP for Islington South and Finsbury. Thus far, the only real blip she has experienced in terms of her parliamentary career was her sacking as shadow attorney general by Ed Miliband after she tweeted a photograph of a house in Rochester that was decorated with three St George’s flags in 2014, during the byelection there (she never deleted the tweet, which is telling). “I’ve always been driven,” she says. “I always joke that I have a chip on each shoulder – that it gives me balance – but actually, being chippy is a good thing. I feel I have a lot to prove. I still remember my careers teacher. I still remember the low expectations.”

Her self-confidence is a hard-won thing; in the beginning, I suspect, she performed it, wearing it almost like a fancy dress costume until, at a certain point, it turned into the real deal. She is still, she thinks, on the receiving end of sexism – “this is a patriarchal society; things aren’t sorted out yet” – but she also believes that it’s better to be honest about this stuff. “I hope I can give support to other women, to tell them they can do it and to ask them to do it,” she says. “Women have to be asked and asked again. Before I stood for shadow cabinet, Harriet Harman chased me down a corridor and shouted at me. That was the only reason I agreed – because I was embarrassed.” Are there still moments when she suffers from impostor syndrome? “Less than I did. I’m getting over it.” She laughs (she’s always laughing). “I hope all that will be gone completely by the time I’m 60.”

• This article was amended on 22 July 2019 to update a reference to a news report about the Labour staff, members and supporters’ letter.