It was December 2020 and Janet (not her real name), a 53-year-old IT consultant from Edinburgh, was doing her weekly shop in the Sainsbury’s closest to her house. In went the toilet roll, meat and vegetables. She wheeled her trolley on to the feminine hygiene aisle and saw a pair of period knickers. They were £12.

Janet frowned and considered. She had heard about period underwear; a friend raved about her Thinx. Janet had even been on the Thinx website, but the cheapest pair, with delivery, were more than £30. “It was quite a lot of money if I didn’t like them,” she says. She was doubtful that the knickers would even work for someone who was perimenopausal and had recently gone back to using sanitary towels for the first time since she was a teenager because she found it too uncomfortable to wear a tampon.

Janet tossed a pair into the trolley. “I thought: £12 is not that extortionate. I’ll buy one pair and give it a go,” she says. “It was very much an impulse buy.” At home, she took them to the bathroom sink and dribbled water on them, “just to have that confidence that they’d absorb the liquid.” She watched, astonished, as the knickers absorbed the fluid entirely. Then she got back in her car and drove off to buy another pair. She has never looked back.

“I was really sceptical at first,” she says. “I thought I’d put a pair on and have to change them, as you would a pad. But there’s no odour and there’s no dampness. And it feels cleaner as well.”

Janet is one of the many women and girls across the UK discovering the joys of period pants. Once a niche product for women with money to spare, period knickers have taken over the UK high street. You can buy them in M&S, Boots, Tesco, Sainsbury’s, John Lewis, Next and Marks & Spencer. Primark’s are the cheapest, at £6 for a single pair or £13 for a three-pack. “These efforts from mainstream retailers will help to drive the accessibility of these formats, which were once considered very niche,” says Emilia Greenslade of the market research firm Mintel.

Period pants fit within a broader trend, namely the disruption of the period industry by startups and entrepreneurs keen for a slice of the £29bn global menstrual market. Whereas once the market was dominated by the big tampon manufacturers, which often marketed products using the language of stigma and shame, women and girls can now sign up to tampon delivery services and purchase products infused with CBD, as well as menstrual cups in any colour you want, but preferably millennial pink. “I call it the modern period,” says Dr Lara Freidenfelds, the author of The Modern Period: Menstruation in Twentieth-Century America. “Women aren’t ashamed of their bodies, but we want to handle our periods in ways that don’t impinge on our lives.”

Period pants represent a new frontier in the modern period, enabling women to menstruate without the inconvenience of messy cups or the environmental waste of single-use items. “As a product, it taps into the market for menstrual products that enable women to handle menstruation so it’s convenient, efficient, never shows and never gets in the way,” says Freidenfelds.

On the whole, period pants have been enthusiastically embraced by Britain’s menstruators. “We’ve sold 100,000 pairs since we launched in April, mostly through our website, but we’re also launching to 70 stores at the end of the year,” says Soozie Jenkinson, the head of lingerie design at M&S. A Primark spokesperson says that the company is seeing “high demand” and will probably develop further styles to add to its 17-piece collection in the coming months. Boots introduced period underwear in June and reported that, over the month, sales increased by 319%.

The explosion in popularity of period underwear is partly fuelled by growing recognition of the ruinous environmental impact of single-use menstrual products. The Women’s Environmental Network estimates that up to 2bn menstrual items are flushed down Britain’s toilets each year. “Consumer interest in reusable sanitary protection products has risen over the last few years, driven by media coverage and campaigns around the impact of disposable tampons and pads on the environment,” says Greenslade. “Brands and retailers are responding to the growing demand.”

According to Mintel’s data, in 2020, 15% of menstruating women opted for environmentally friendly products. Given that UK retail sales of intimate hygiene, sanitary protection and adult incontinence products are projected to grow by 11% to £544m in 2025, it is unsurprising that retailers are raising their game. Even the sanitary towel manufacturer Bodyform now makes period knickers – a savvy move, given the growing backlash against sanitary towels, which typically end up in landfill.

“I just think there is such a huge waste problem – and people using things just once, whether it’s plastic tampons or fast fashion, really gets on my nerves,” says Georgia, a 23-year-old researcher from south London. She had wanted to try period pants for a while, but couldn’t afford Modibodi or Thinx, which hover around the £25 mark. But, in June, a friend told her that Primark was selling three-packs for £13. “I couldn’t believe it,” she says. “That was less than for one pair of Modibodi.”

Her intention in buying the cheaper knickers was “to test them and see how I get on, then buy the expensive ones if I like them”, Georgia says. But despite the low price point, her Primark knickers have so far held up without leaking. “They’re great,” she says. She has even given pairs to friends as a gift: “I have one friend who was trying to be more sustainable, so I bought her period pants and a plastic-free deodorant.”

The word “sceptical” comes up frequently when discussing period underwear, with many women doubting that the underwear will hold up. But, remarkably, even the cheapest high-street offerings seem to do the job. “I probably wouldn’t wear them with a white dress on the heaviest day of my flow,” says Heather, a 32-year-old author from Edinburgh. “But I don’t know if I’d wear a white dress with any period product. In general, I’ve not had any problems with leakage.”

Heather bought her Bodyform knickers for £25 from Tesco three months ago. “I was interested in getting some for ages, but they were only available through startups, niche websites, and for some reason that was a block for me,” she says. “Because they were Tesco, I thought: it’s just part of my shop.” She describes herself as “evangelical” about the pants. “You never feel wet,” she says. “And it doesn’t feel like you’re wearing anything bulky.”

For some women, the home working occasioned by the pandemic has given them the opportunity to road-test period pants. “Because I was working from home, I was able to be experimental without mishaps and embarrassment,” says 26-year-old Shomi Williams, a therapist from Edgware, London, who bought Modibodi knickers online (they are now available at Next). For Williams, the environmental factor is a boon, but she says: “I’m not going to pretend sustainability is a primary factor.” What appeals is their comfort: “I’m not aware of their presence. They feel like regular pants.”

While period knickers vary from brand to brand, their basic design is the same: a cotton or polycotton blend, with an ultra-high-absorbency fabric sewn into the gusset. Users simply rinse out the knickers in cold water, before putting them in the washing machine along with a regular load of laundry. (Users are advised to wash the knickers at low temperatures, because the synthetic fibres used in the gusset can degrade into microplastics and ultimately leach into our oceans.)

Jenkinson says M&S period knickers have a “three-layer gusset system”: a layer next to the skin that draws away moisture so that wearers feel dry; a middle layer that absorbs blood; and an outer layer with an anti-leak membrane. The greatest challenges, she says, were “ensuring the knickers stayed secure without feeling bulky and making sure that you could move really well. And making sure the garments looked feminine – we wanted them to look like everyday knickers.”

Unsurprisingly, producing period pants at affordable prices, at mass scale, is difficult. Primark’s range took 18 months to conceive and bring to market; M&S’s took about a year. “It’s about clever design and working with the technology team to create an innovative and affordable product,” says Jenkinson. Scale helps to keep manufacturing costs low. “We are a global company. We sell millions of knickers every year to our customers.”

Although the technology behind period pants is new, the principle is not. Williams, who is British-Nigerian, points out that, in Nigeria, reusable baby nappies are commonplace. “In terms of the bigger picture, women have been using a much less sophisticated form of reusable menstrual pads for much of history,” says Freidenfelds.

Single-use menstrual products emerged in the US in the early 20th century: Kotex introduced sanitary towels in the 20s, while Tampax began manufacturing tampons in the 30s. “That was a disruption,” says Freidenfelds. “It transformed how women handled periods.” Disposable menstrual products liberated women from the tedium of having to wash out menstrual rags. After many years, we have come full circle, albeit with the benefit of 21st-century washing machines.

“I don’t see period pants as a major break with the past,” says Freidenfelds, adding that the manufacturers have improved the technology. Earlier forms of environmentally friendly period products were uncomfortable or difficult to use: menstrual cups are fiddly and reusable pads often stay wet during the day. Before buying period pants, Georgia used reusable pads that she tucked inside her knickers. They were environmentally friendly, but “sometimes you could feel them when you sat down or were walking around,” she says.

By contrast, period knickers have “got the tech to a place where it can compete with an Always pad with wings,” Freidenfelds says. This is crucial, because, in her experience, women “are not really willing to compromise and say that a leak here and there is acceptable because they’re environmentally friendly.”

Jenkinson has worked hard to make M&S’s offering look like regular knickers – and they do. The fabric is thicker than usual around the gusset, but not uncomfortably so. (Primark’s offerings are more nappy-like – although, given that they are less than half the price, this seems reasonable.) But making them comfortable and alluring is a tall order. “I call them old-women knickers,” says Janet, frankly. “They’re not stylish.” She recently bought some sportier-looking period pants from Amazon, to wear at the gym. “They’re a bit funkier,” she says.

The appeal of period pants is not only limited to adult women. Learning to use a tampon is daunting for many girls, while pads can be problematic in various ways, says Freidenfelds. One mother tells me that, after her daughter began menstruating at 10, she refused to go to school on the days she had her period. Two years later, she happily wears period knickers to school.

Period pants can also be transformative for women experiencing incontinence, fibroids or menopausal flooding, or those who prefer not to use tampons for cultural or religious reasons. “Being menopausal, I get a tiny bit of stress incontinence when I run on the treadmill. It was putting me off exercising,” says Janet. Period knickers are an unobtrusive solution to incontinence pads, particularly in communal changing rooms. “If someone sees me taking my leggings off, they look like normal underwear,” Janet says.

Sharon (not her real name), a 52-year-old business owner from Surrey, says her Modibodi period pants have been “a godsend”. Sharon has fibroids, which means her periods are extremely heavy. To sleep, she used to have to wear two sanitary pads and lie on top of a towel, but even then she would often stain the sheets. “You’re waking up constantly in the night to make sure the pad is in the right place, so you’re knackered,” she sighs. She couldn’t take the dogs for a walk for longer than an hour without flooding through her clothes. “I always had to have a sweatshirt with me to wrap around me, just in case.”

Apart from longer walks with her dogs, she says: “I can go on trains – I don’t have to drive everywhere, in case I have an accident. I can sleep without putting down a towel. And I feel more confident. I know that sounds ridiculous. It’s such a weight off me.”

Listening to Sharon talk about the dramatic impact period knickers have had on her life, it seems arbitrary and unfair that it has taken until 2021 for such a gamechanging technology to be made widely accessible at affordable prices. Many of the women I speak to echo a similar sentiment. “It annoys me,” Heather says, laughing. “These are so great. How have they only just become available?”

• This article was amended on 1 September 2021 to replace a reference to Mooncups with “menstrual cups”; Mooncup is a trademark brand rather than a general name for such products.