THANKS, EARL (profanity alert):

The invention of the chilled packaged sandwich, an accessory of modern British life which is so influential, so multifarious and so close to hand that you are probably eating one right now, took place exactly 37 years ago. Like many things to do with the sandwich, this might seem, at first glance, to be improbable. But it is true. In the spring of 1980, Marks & Spencer, the nation's most powerful department store, began selling packaged sandwiches out on the shop floor. Nothing terribly fancy. Salmon and cucumber. Egg and cress. Triangles of white bread in plastic cartons, in the food aisles, along with everything else. Prices started at 43p.

Looking upon the nation's £8bn-a-year sandwich industrial complex in 2017, it seems inconceivable that this had not been tried before, but it hadn't. Britain in 1980 was a land of formica counters, fluorescent lighting and lunches under gravy. Sandwiches were thrown together from leftovers at home, constructed in front of you in a smoky cafe, or something sad and curled beneath the glass in a British Rail canteen. When I spoke recently to Andrew Mackenzie, who used to run the food department at M&S's Edinburgh store - one of the first five branches to stock the new, smart, ready-made sandwiches - he struggled to convey the lost novelty of it all. "You've got to bear in mind," he said. "It didn't exist, the idea."

If anything, it seemed outlandish. Who would pay for something they could just as easily make at home? "We all thought at the time it was a bit ridiculous," said Mackenzie. But following orders from head office, he turned a stockroom into a mini production line, with stainless steel surfaces and an early buttering machine. The first M&S sandwiches were made by shop staff in improvised kitchens and canteens. Prawns defrosted on trays overnight, and a team of five came in before dawn to start work on the day's order.

And, oh, they sold. They sold so fast that the sandwich experiment spread from five stores to 25, and then 105. Soon, Mackenzie was hiring more sandwich makers in Edinburgh. In the Croydon branch, a crew of seven was making a hundred sandwiches an hour. The first official M&S sandwich was salmon and tomato, but in truth it was a free-for-all. They sold so fast that staff made them out of whatever was lying around. In Cambridge, they made pilchard sandwiches, and people wanted those, too.

Without being designed to do so, the packaged sandwich spoke to a new way of living and working. Within a year, demand was so strong that M&S approached three suppliers to industrialise the process. (One of the world's first sandwich factories was a temporary wooden hut inside the Telfer's meat pie factory in Northampton.) In 1983, Margaret Thatcher visited the company's flagship store in Marble Arch and pronounced the prawn mayonnaise delicious.

Every supermarket jumped on the trend. Up and down the country, chefs and bakers and assorted wheeler-dealers stopped whatever they were doing and started making sandwiches on industrial estates. The sandwich stopped being an afterthought, or a snack bought out of despair, and became the fuel of a dynamic, go-getting existence. "At Amstrad the staff start early and finish late. Nobody takes lunches - they may get a sandwich slung on their desk," Alan Sugar told an audience at City University in 1987. "There's no small-talk. It's all action." By 1990, the British sandwich industry was worth £1bn.

A young economics graduate named Roger Whiteside was in charge of the M&S sandwich department by then. As a young buyer, Whiteside had come up with the idea of a set of four peeled oranges, to save customers time. He had read that apartments were being built in New York without kitchens, and he had a sense of where things were going. "Once you are time-strapped and you have got cash, the first thing you do is get food made for you," he told me. "Who is going to cook unless you are a hobbyist?"

In the sandwich department, he commissioned new prototypes every week, and devised an ultimately impractical scheme to bake baguettes in west London each morning and deliver them, still crusty, to stores around the capital. Baguettes go soft when they are refrigerated - one of a surprising number of technical challenges posed by sandwiches. Whiteside immersed himself in questions of "carriers" (bread), "barriers" (butter, mayonnaise), "inclusions" (things within the bread), "proteins" (tuna, chicken, bacon) until they bordered on the philosophical. "What is more important, the carrier or the filling?" he wondered. "How many tiers of price do you offer in prawn? How much stimulation do people need?"

In the early 90s, Whiteside developed M&S's first dedicated "food to go" section, with its own tills and checkouts, in Manchester. The innovation prefigured the layout of most contemporary supermarkets, and was fabulously successful. But it wasn't successful enough for Whiteside. He didn't understand why absolutely everyone in Manchester city centre wasn't coming in to M&S for their lunch.

One day, he went into a branch of Boots on the other side of the street. Like almost every major retail chain, the pharmacy had followed M&S into the sandwich business. (Boots established the country's first national distribution system - selling the same sandwiches in its all branches - in 1985, and pioneered the meal deal.) But Whiteside was convinced that its sandwiches weren't as good as M&S's, and that most customers knew that, too. He confronted the lunchtime queue in Boots and asked people why they weren't coming to his store. "They said: 'Well, I am not crossing the road'," he recalled.

The answer struck Whiteside with great force. Mass-producing a meal that you could, if necessary, rip open and consume in the street was transforming people's behaviour. "Instant gratification and total convenience and delivery," Whiteside said. "If you are not there, they are not going looking for you." He returned from Manchester and tried to persuade M&S to open hundreds of standalone sandwich shops in London. "It was so obviously an opportunity." M&S didn't go for the idea, but Whiteside was convinced that the future would belong to whoever was selling on every corner. He saw Pret and Starbucks and Costa and Subway coming a mile off. During the 1990s, the sandwich industry trebled in size. By the end of the 20th century, more people in Britain were making and selling sandwiches than working in agriculture.

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