“You are in chaos,” Margaret Atwood decrees calmly over tea. “It’s Cavaliers versus Roundheads.” We meet in London, the afternoon before she is due at Waterstones in Piccadilly to sell the first copy of The Testaments, the long awaited sequel to The Handmaid’s Tale, at midnight, accompanied by women in the now globally recognised handmaids’ costumes. Further up the river, Westminster is shutting down for five weeks. Ever since Boris Johnson raised the prospect of proroguing parliament, people have been sharing a quote from The Handmaid’s Tale, published 35 years ago, just as they did in the US following the election of Donald Trump in 2016: “That was when they suspended the Constitution. They said it would be temporary. There wasn’t even any rioting in the streets. People stayed at home at night, watching television.”

“It is another predictable mess that people have got drawn into because a big honking pack of lies was told about it,” Atwood says of Brexit. Apart from the legal questions – “actions have consequences, but what will the consequences be?” – she is most interested in “who is going to benefit from this financially? Who is shorting the pound? I can think of all kinds of people who might turn it to their advantage.”

As has been noted, the last time a novel received such a fanfare was the midnight launches for Harry Potter books – with fans sporting specs and scarves instead of capes and bonnets. Atwood is our high sorceress, a very grownup one, with a truly unnerving knack for auspicious timing. Her best work combines psychological acuity with audacious curiosity; she is as good at recreating the politics of the playground (Cat’s Eye, 1989) as she is at inventing a chillingly plausible male theocracy. She is both deadly serious and surprisingly playful, and it is to her that readers have turned as a way of making sense of the world, helped by the hugely successful TV adaptation of The Handmaid’s Tale. With cameo appearances as an Aunt, and another in the TV version of her 1996 novel Alias Grace, which also aired in 2017, she describes the last couple of years as “horrible fun: when the occasion for the fun is not so happy. And the occasion for the fun is the rise of religious fundamentalist actions against women”.

Facebook Twitter Pinterest People dressed as handmaids queue to get a copy of The Testaments on the day of its release. Photograph: Dylan Martinez/Reuters

Like Kafka and Orwell, Atwood has become part of the public discourse; the red and white robes adopted as a symbol of female defiance from Ireland to Argentina, but most often in the US, where her near-future dystopia, in which women have been reduced to female reproductive slaves, was set. Yet back in 1985, when The Handmaid’s Tale, Atwood’s sixth novel, was published, “a big fuss was not particularly made. I think we had some sort of publisher’s party,” she says dryly.

The sequel has been surrounded by levels of secrecy to rival any ex-prime minister’s memoir, with tales of Booker judges’ computers being hacked (The Testaments is on this year’s shortlist). “I left my computer on a plane,” she confides. “I forgot about it because I was watching Captain Underpants.” She didn’t have a copy of The Testaments on it, but there was enough information to cause a stir if it fell into the wrong hands. Atwood doesn’t – yet – have the powers to ground a plane, but she was reunited with her computer within two hours. “It would have been a catastrophe,” she says, laughing now.

Then Amazon blew it all by sending out copies in the US a week ahead of publication. “Somebody pushed the wrong button. It was human error,” she says unruffled. “They apologised, which is apparently among the first times, if not the first time, that Amazon have ever apologised.”

Despite her formidable energy, Atwood embodies an owl-like stillness – head tilted, listening carefully, with only her hands moving when she speaks. She gives long, far-ranging answers (not always to the question asked), and can leap from Mary Queen of Scots to Mary Poppins, or from the Cromwells to the day’s news, in the blink of a gimlet eye. At nearly 80, what does she make of this remarkable late boost? “Isn’t it wild,” she says. “I can’t remember a time when I was a usual anything,” she says. “I mean going way back. I didn’t grow up in a usual way” (her childhood was spent in the Canadian wilderness). “I wasn’t a usual kind of high-school student. I was a very peculiar university student and it continued on from there.” Atwood “became” a writer at 16, when she made up a poem in her head while crossing a football field. Since then she has written more than 60 books, in almost every genre, and has been awarded more than 100 prizes, including the Booker in 2000 for The Blind Assassin. Her futuristic trilogy – Oryx and Crake (2003), The Year of the Flood (2009) and MaddAddam (2013) – has an ardent following. She lives in Toronto, spending her summers on Pelee Island, where she has written many of her novels, including much of The Testaments (as well as a stint, more surprisingly, in Norwich).

This is a hinge mom­ent in our times. One of those puzzles in which you need to open the right door. So, human race, which door will you open?

The author has just had a long chat with Bruce Miller, the TV series’ showrunner, who is “quite thrilled” about the new novel, “because it gives him a lot more to work with”. Atwood, an executive producer on the show, had started writing The Testaments before the first season launched. And there shouldn’t be any clashes, as her sequel picks up 15 years later: “We know where we start in the 15 years, but there’s quite a lot of squeezing room for things that happen leading up to that,” the author explains. “I’ve given him a lot of blank space that will be his to fill. We agreed he can’t kill off certain people a long time ago.”

“Hands off Aunt Lydia” and “don’t touch that baby” were her principal stipulations. “I needed this child here and that child there – so that’s how it’s going to be.” The Testaments is narrated by three voices: it is the last will and testament of Aunt Lydia, and the testimonies of two girls, Agnes who has grown up in Gilead, and Daisy who escaped to Canada with her mother as a baby. Atwood’s reluctance to write as Offred, the narrator of The Handmaid’s Tale, was one of the reasons it took her so long to attempt a sequel, despite “the clamouring” from readers. “I couldn’t do it,” she says emphatically. “Not possible. To recreate that voice and do more of it. I just couldn’t.” She is wary of follow-ups in general. “They are always kind of imitations. I saw Mary Poppins Returns. A commendable sequel. I enjoyed it, but it’s not the same.”

Atwood has said in the past that The Handmaid’s Tale was written to answer the question: if there was a totalitarian regime in the US what form would it take? The Testaments sets out to show how such a regime collapses: “What makes them turn into something else? What makes them fall apart?” And for that she needed an insider: “So who in that whole cast of characters is going to know more? There’s only one answer to that.” Aunt Lydia, a female Thomas Cromwell, “swollen with power” but without absolute authority.

Facebook Twitter Pinterest ‘We will not forget’ ... protesters dressed as handmaids stand outside the US supreme court in 2018. Photograph: UPI/Barcroft Images

Many readers have wondered what they might do in Offred’s situation, how to resist and survive; by giving us Aunt Lydia’s story, Atwood asks us to think about questions of cruelty and complicity. All her novels are about survival. As Aunt Lydia reflects: “What good is it to throw yourself in front of a steamroller out of moral principles and then be crushed flat like a sock emptied of its foot? ... Better to hurl rocks than to have them hurled at you.” Or as Atwood puts it: “We are ordinary as long as it is easy to be ordinary.” She quotes a Polish resistance fighter from the second world war who once told her: ‘“Pray that you will never have the occasion to be a hero.’ You don’t run into a burning building and rescue somebody unless there’s a burning building … Similarly, you don’t run away from a burning building and ignore everything, unless there’s a burning building.”

But the main reason for not returning sooner to the world she had created was because, before now, it no longer seemed relevant. “For a long time we were going away from Gilead and then we turned around and started going back towards Gilead, so it did seem pertinent.” The Handmaid’s Tale, which Atwood began writing in West Berlin in 1984, was her response to those who wanted to reverse the freedoms gained by women in the 70s, to put the butterflies back in the box. “They just didn’t have the power to make that happen.” Now they do.

The novel’s status as a feminist classic has perhaps overshadowed another of its most prescient warnings – the catalyst for Gilead was environmental catastrophe. In The Testaments Aunt Lydia catalogues the natural disasters that ravaged “her vanished country”, before continuing. “People became frightened. Then they became angry. The absence of viable remedies. The search for someone to blame. You don’t believe the sky is falling in until a chunk of it falls on you.”

If the novel deals less explicitly with the crisis than we might have expected, her point is the same: the burden will fall hardest on women and children. “It always does, because women and children are weaker – let’s take their stuff,” she says. But, as Atwood points out, she “did climate apocalypse quite thoroughly” in the MaddAddam trilogy. “Maybe not thoroughly enough,” she says, “because I think it is actually going to be worse.” But she is not without hope, and if she was younger she would “absolutely be doing Extinction Rebellion”. She was the first writer to donate a manuscript to the Future Library, in which stories remain unread for 100 years, when they will be printed on paper from a newly planted forest in Norway.

At an event last summer, a member of the audience asked Atwood for one word to sum up the times we are living in. “Vital,” she fired back. “This is a hinge moment, in many different ways,” she says now. “Let’s call it the casket moment,” an allusion to the three caskets in The Merchant of Venice. “One of those puzzles in which you need to open the right door. So, ‘All right human race, which door are you going to open?’”

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One of Atwood’s central axioms is that nothing can be included in her fiction that hadn’t happened somewhere in the world. As Atwood has reminded us: she didn’t make this stuff up, the human race did. “If I was to create an imaginary garden, I wanted the toads in it to be real,” she wrote in a recent introduction to The Handmaid’s Tale. And this is “absolutely” the case for The Testaments: shadows of Weinstein and Epstein, Isis and the Trump administration can all be found in its murky waters. It is impossible to read the scenes set in a female detention camp without thinking of the US-Mexico border.

Warnings aren’t effective if they are just preaching.

Is Atwood firing a warning or reflecting what is already happening? “Both. But warnings aren’t effective if they are just preaching. Enactments are of interest because then people think ‘What would I do?’ Whereas if someone is going ‘Don’t do that, don’t do that,’” she says, wagging her finger. “You think ‘Oh shut up!’”

This is the sixth time Atwood has been shortlisted for the Booker prize. “There are upsides and downsides” to winning, she says. If you win everyone says you shouldn’t have won the Booker. And if you don’t win, in Canada anyway, they say ‘Atwood fails to win Booker’.” And then there is the Nobel. “Oh purlease …” She is already at work on a collection of poetry, often her way of working up to a novel. And she hasn’t ruled out a further instalment: “Never say never to anything.”

“Only dead people are allowed to have statues, but I have been given one while still alive”: so Aunt Lydia opens The Testaments. Is there an Atwood statue? “There’s a head,” she laughs. “We used to call it the horrible head.” The plaster cast is in her cellar, the original, made in the 70s, at McMaster University. “I don’t look like that,” she told the sculptor, who replied. “But you will …”

“People bang on about your legacy,” she says, impatiently. “I’m not that interested because I’m not going to be here. I’m not going to be around haunting my legacy. Unless I do an Aunt Lydia and bury little manuscripts in libraries.” Well, there’s already that story buried away in the Future Library. Praise be.

• The Testaments by Margaret Atwood is published by Chatto (£20). To order a copy go to guardianbookshop.com. Free UK p&p on all online orders over £15.