Oliver Letwin’s strange and somewhat alarming new book begins at midnight on Thursday 31 December 2037. In Swindon – stay with me! – a man called Aameen Patel is working the graveyard shift at Highways England’s traffic HQ when his computer screen goes blank, and the room is plunged into darkness. He tries to report these things to his superiors, but can get no signal on his mobile. What’s going on? Looking at the motorway from the viewing window by his desk, he observes, not an orderly stream of traffic, but a dramatic pile-up of crashed cars and lorries – at which point he realises something is seriously amiss. In the Britain of 2037, everything, or almost everything, is controlled by 7G wireless technology, from the national grid to the traffic (not only are cars driverless; a vehicle cannot even join a motorway without logging into an “on-route guidance system”). There is, then, only one possible explanation: the entire 7G network must have gone down.

It sounds like I’m describing a novel – and it’s true that Aameen Patel will soon be joined by another fictional creation in the form of Bill Donoghue, who works at the Bank of England, and whose job it will be to tell the prime minister that the country is about to pay a heavy price for its cashless economy, given that even essential purchases will not be possible until the network is back up (Bill’s mother-in-law is also one of thousands of vulnerable people whose carers will soon be unable to get to them, the batteries in their electric cars having gone flat). But Apocalypse How? is not a novel. It’s a peculiar hybrid: part fable, part fact. Aameen, Bill and all Letwin’s other characters exist only to illustrate aspects of his wider thesis, which is that our increasing reliance on integrated digital technology may be leading us, and ultimately every country in the world, in the direction of a catastrophe. I exaggerate a little, but think TV’s Survivors minus the mystery virus (though at the moment, we handily have one of those on our hands, too).

Letwin, an old fashioned keep-calm-and-carry-on kind of Tory (though he left parliament at the 2019 general election), doesn’t look or sound like a doom-merchant. Even last year, as he led the bloody parliamentary fight to ensure that Britain did not leave the EU without a deal, he was always smiling: a cherubic figure whose springy hair – it rises benignly from his head like ice-cream from a cone – only made him seem all the more graciously donnish. But a doom-merchant he is, or so it would seem. All he needs now is a sign that reads: “The end is nigh”.

The thoughts of rack and ruin started when he was still a member of the cabinet (his last big job was chancellor of the duchy of Lancaster under David Cameron). “In government, I’d be dealing with all sorts of risks,” he says. “Each day, I put aside a small bit of time in which to think, and on one of these occasions, I forced myself to consider how likely it was that any future administration would be able to protect us from certain risks to a degree that we could be fairly confident the situation in question wouldn’t arise. The more I thought about it, the more it became clear there wasn’t any serious likelihood of that at all. This lodged itself in my mind, and it worried me for months and months.” Apocalypse How? is the 220-page result of all this worry. In its pages, a space-weather event having taken its effect on satellites, Britain falls into chaos for five days, when, to a degree, law and order breaks down and 100,000 vulnerable people die (frail, cold and hungry, neither the emergency services nor the army can reach them, there being no functioning communications system, and no paper record of their whereabouts).

Letwin and I meet in Mayfair, at the Legatum Institute, a right-leaning thinktank where he is now a fellow. It’s a luxurious place, halfway between a gentleman’s club and a Harley Street clinic. Sitting in reception, magazines fanned out on a coffee table, I almost expect him to appear before me in a white coat, a stethoscope dangling at his neck. But of course it’s me who’ll be asking the questions, nodding sympathetically as he lets it all out. Does he like to talk? He does. Letwin is, I think, used to being listened to, a privilege afforded him ever since he was a small child (we’ll come back to this). His answers are charming, but prolix. His way of dodging a bullet is to fill the air with words, and never mind if they refer to another matter entirely.

Before we get on to the internet, and its terrifying powers, I must ask: what’s it like on the outside? Is he in mourning for life as an MP – he entered parliament in 1997, as the member for West Dorset – or does he feel like dancing across Berkeley Square? He grins. “If you want to have a glimpse of what entering the garden of Eden is like, what you need to do, in my experience, is be an MP for a quarter of a century and then stop. This is just… totally wonderful. I can’t describe how marvellous it is. I very much enjoyed my time in parliament, though I can’t say I enjoyed the last year. But the first morning of waking up, and not having several hundred emails to deal with… I hadn’t realised how wonderful the commodity of time is.” He laughs, a high-pitched giggle that makes me think of a baby that has just seen itself in a mirror for the first time.

Facebook Twitter Pinterest Oliver Letwin speaking as an independent MP in the Commons on 19 October, 2019, during a crucial debate on the Brexit deal. Photograph: Jessica Taylor/UK PARLIAMENT/AFP via Getty Images

And how does he feel about the country, post-election and post-Brexit? (He was for Remain in the referendum, but he also voted for all Theresa May’s deals and for the one Boris Johnson got through the Commons.) Has it, in some profound way, changed? “That’s a big question,” he says. “Lots of things happen simultaneously in countries; they’re very complicated. But I do think there is a national consensus that we’ve done what we’ve done, and we must now try and get on and be sensible.” He hopes negotiations with the EU will take place in a more “sane and rational” environment – though this isn’t to say that he thinks our trading relationship will be easy to sort out. “It could go well, or it could go… less badly,” he says, uncertainly.

Did he fall out with anyone over Brexit? Does he, for instance, feel cross with his old friend David Cameron? “No, not at all!” he yelps. “I’m a huge admirer of his. A referendum was inevitable; it would have created a real problem for our democracy if we hadn’t had one.David was a great prime minister. And I didn’t fall out with anyone else either. I wasn’t cross with anyone, and I didn’t shout at them. Some of my closest friends are Brexiters, like my best man, Charles Moore [the former editor of the Daily Telegraph]. What I do regret, though, is that some people were so annoyed with me [over his determination to frustrate no-deal] that they became quite, er, hostile.” Did he feel plucky, taking such a stand? He tuts. “No! This is not a place where you get shot or imprisoned for political disagreements. It required perseverance, that’s all.” Perseverance, it would seem, comes easily to him. His next book, about the rise of the east and how the west should deal with it, will cover – gulp – some 4,000 years of history.

But we must talk about the book I’m holding in my hands: Apocalypse How? It all began, he thinks, with his wellington boots, which he had to pull on with alarming regularity while in government. “I don’t know if you remember the terrible floods that besieged the Lake District during the coalition government? Well, when we finally got through those, I sat down with senior experts in the field, and I asked them: why are these events that are supposed to happen every year in 100 happening each year? And in that moment, I could see it in their eyes. How, they thought, can this minister be so ignorant?” He laughs.

“Very patiently, they began to explain to me what should have been blindingly obvious. If you have 1,000 places that have a one in 1,000 chance of flooding, it’s pretty likely one will flood every year – and even more so if they have a one in 100 chance of flooding. The statistics don’t mean what the layman thinks. If we’re going to avoid flooding, we must assume it is going to happen.”

If I hear there's a problem about fuel, I fill up. I have a paper record of my contacts. These are sensible precautions

There was something else, too: the question of networks. “I was conducting resilience reviews. I’d assumed the various networks essentially to be separate from one another. But as I explored this, it became clear that within not too many years, there would essentially be only one. The internet of things [the billions of devices that will eventually be connected to the internet, from fridges to dustbins] is the revolution that’s coming, and it’s a tipping point: a lot of information is converging, everything that we depend on.” Converging networks are fragile; they can be brought down, in a single stroke, by a space-weather event or sabotage, whether physical (explosives, say) or cyber. And as more and more parts of our lives depend on fewer and fewer of these networks, society becomes ever more vulnerable. This isn’t, he insists, science fiction; it’s a coming reality. Letwin believes we have only to look at Russian cyberattacks on Ukraine’s power grid in 2015 and 2016 to see this.

Surely someone, somewhere, is on top of this? Apparently not. According to Letwin, politicians are unwilling to do things for which they won’t be thanked (if the public was successfully protected from digital meltdown, no one would ever know, and thus their efforts would go unacknowledged). They operate on a short-term basis, and they procrastinate. Cost-benefit analysis combined with the difficulty of estimating probabilities is likely to suggest to them that investing in this area simply isn’t worth it. Like all of us, they also tend, for complex psychological reasons, to underprepare for disaster. Needless to say, I’m transfixed by his honesty in this matter – as if he had not been a career politician himself! The irony, however, seems to elude him.

Of course, as he also notes in his book, policymakers could bring in “free thinkers” to “think the unthinkable”. But this is tricky. Dominic Cummings’s dim view of special advisers, and his call for “weirdos and misfits” to apply for jobs at No 10, don’t quite wash with Letwin. “I understand why people get frustrated by bureaucracies. There are moments when you’re facing a very simple crisis, one so great you must just cut a swath through it. The second world war would be an example of that, when defeating the Nazis was the only thing that mattered, and Churchill got this in a way other people didn’t. But that’s only appropriate when the threat is immediate and existential. Most things are more like climate change than the Nazis; they’re not simple, they’re not immediate, and you can’t act alone.”

Facebook Twitter Pinterest Margaret Thatcher lends her support to Letwin’s 1992 parliamentary bid. Photograph: The Independent/Rex/Shutterstock

All this said, you have to wonder why heads are buried so deeply in the sand when it comes to our digital future. By his telling, the solution isn’t difficult. “What it amounts to is an analogue solution to a digital problem,” he says. “It wouldn’t be a perfect back-up, but it would get us through. We need to ensure that stuff we could do 50 years ago has been laid in store in such a way that we can get by.” He’s talking, I think, about old-fashioned things such as landlines, walkie-talkies and, perhaps, reserves of diesel and the kind of robust vehicles that require it.

Naturally, I can’t help wondering if all this has affected his own life. Is his house awash with torches, tins and clockwork radios? Is he a secret survivalist? “Well, we have four or five torches. Which work.” He giggles. “And if I hear there is a problem about fuel coming along, I fill up. I have a map in the car, and a paper record of my contacts. These are sensible precautions.” Later, I make a joke about yoghurt pots tied together with string. But it falls flat, possibly because at his primary school, unlike mine, children were not taught how to make rudimentary communication devices from household debris.

What does he make of the government’s decision to allow the Chinese company Huawei to serve as a supplier to create a new 5G network in Britain? Many Conservatives believe this will put national security at risk. I expect alarm, but he is entirely (and, to me, bizarrely) unperturbed. “I’m clear there are very good reasons, in terms of our security, why we should have Huawei. The fact that as part of the deal we have transparent understanding of what they’re doing, makes us less exposed, it seems to me, to the Chinese.” It is concerning that the world hasn’t yet found a way of establishing international governance of cyber-security, but he has little to say on the question of Russian interference in the EU referendum. “I haven’t the slightest idea [whether the Russians meddled in it]. I think it is extraordinarily silly of people to speculate.” Should the government clear the release of the House of Commons’ intelligence and security committee’s report into Russian infiltration? “I don’t know. If it contains things it’s important the government knows, but equally important other people don’t know, then they shouldn’t.”

Eton is a privilege and a millstone. It creates an often mistaken impression of what you are, and that’s unfortunate

Letwin is not precisely what many people perhaps imagine him to be, for all that he is an Old Etonian with a voice to match. (“Oh, I see,” he says, when I point this out. “You thought I was a seventh earl or something.”) His grandparents were Ukrainian Jews who fled Europe for America. His parents, William and Shirley, were American academics who settled in Britain (his father was professor of political science at the London School of Economics; his mother taught, in the 70s, at that hotbed of radical conservative thought, Peterhouse College, Cambridge). They were also famous hosts, whose salons – they lived in a house near Regent’s Park – were attended by the likes of Isaiah Berlin.

Letwin was their only child. What was it like growing up in such an atmosphere? “Oh, it was absolutely wonderful,” he says, his voice rising. “It was somewhat chaotic. I spent my first 12 years oscillating between the UK and the US; my parents had academic jobs on each side of the Atlantic in rotation. But I was incredibly lucky. They were enormously lively and endlessly interested in everything, and they had an enormous number of friends and acquaintances. What made it all the luckier was that they were generation blind. The young, the middle-aged and the old were all around the table together, and from a ridiculously early age, I was allowed to be there. Nobody tried to stop me talking. I must have seemed completely ridiculous, but they put up with me. The result was that I had the kind of conversations you usually don’t have as a child: furious arguments with Milton Friedman [the Nobel prize-winning economist]. That was fantastic, and it taught me that it doesn’t matter two hoots how grand someone is – either what they are saying stands up to scrutiny, or it doesn’t.”

Why did his parents send him to Eton? “My mother was completely in love with England, with a romantic dream of it that has largely disappeared. Eton was part of that.” Is it true he was the cleverest boy there? In Nicholas Coleridge’s recent memoir [Coleridge, the former boss of the magazine publisher Condé Nast, was his contemporary], he says Letwin was so bright, people struggled to understand him. “Knowing Nicholas, and loving him as I do, I imagine that book is very romanticised, and doesn’t bear much relation to reality. I was perfectly comprehensible, and simple, and not nearly as clever as some. I was at school with Noel Malcolm [the journalist and historian]. He was at least 10 times cleverer than me.” Still, his reputation for a certain kind of cleverness stayed with him. In the run-up to the 2010 general election, one commentator described him as the Conservative party’s Gandalf.

From Eton, he went to Cambridge University, Princeton and then Cambridge again, after which he joined Margaret Thatcher’s Policy Unit – and thus began his somewhat gaffe-prone political career (among other things, in 2011 he was found to have thrown away confidential letters from constituents in a bin in St James’s Park – a breach of data protection for which he apologised). Is he, I wonder, surprised that our latest prime minister is an Etonian? Shouldn’t things have changed a bit (a lot) by now? “Well, yes,” he says, with a laugh. “I didn’t vote for him to be leader! The thing about Eton is that it is a privilege and a millstone. It creates an often mistaken impression of what you are, and that’s unfortunate. On the other hand, it gives you a fantastic education, and it encourages you to be confident, to take up positions, and not to be intimidated by authority. But it is a kind of shortcut, and an unfair privilege, there’s no doubt about that.”

Predictably, we run out of time at the Legatum Institute. This part of our conversation takes place later, Letwin talking to me on his (yes) mobile phone as he walks to work – a situation that makes it all the easier for me to ask, rather crossly: are people like him on their way out? Will they ever be on their way out? As ever, though, he’s gracious, mild-mannered. “Yes, I think we are, and that is quite right. Society is dramatically more meritocratic and less old school tie than when I was growing up – though we’ve got some way to go.” Has he learned to check his privilege? He insists that he has. “I’m extremely conscious that in a thousand ways I’ve been fantastically lucky,” he says. “I bless my luck all the time – and I know that it is luck.” After we hang up, I picture him beetling along the London pavements, his trouser pockets fat with matches, peanuts and Kendal Mint Cake. An amusing, and slightly terrifying, thought.

• Apocalypse How? by Oliver Letwin is published on 5 March by Atlantic (£14.99). To order a copy go to guardianbookshop.com. Free UK p&p over £15