A traditional chestnut-coloured unisex leather satchel, made in the UK by a mother-and-daughter team, has unexpectedly taken the fashion world by storm. The Cambridge Satchel Company has become 2011's accidental multimillion-pound global hit, and is competing for popularity with international mega-league brands.

The business, which was set up in 2008 by Julie Deane and her mother, Freda Thomas, has gone from selling three of the traditionally styled schoolbags a week from their kitchen table to 1,500 a week, in a multitude of colours.

Turnover has increased from £15,000 to £3.3m, and may reach £8m this year.

The bags have become a cult among twenty-something fashion bloggers, who post incessantly, swapping information about styles and colours. They have become a phenomenon that has surprised the unlikely fashion mogul behind the business.

"I honestly thought my market would be schoolchildren," says Deane, who started the company to pay her daughter's school fees. "She was being bullied. I'm very logical, and worked out the top 10 jobs I could do that would allow me time to still be a mum and would enable me to have my dog with me.

"I was quite obsessed with satchels at the time, because I was reading Harry Potter to my children and I wanted to find them, because they are exactly what Harry and Hermione would have used at Hogwarts."

Deane scoured eBay, stalked traditional school outfitters to find a producer and then sourced leather manufacturers before finding an about-to-retire craftsman in the north-east who agreed to produce her first samples.

Demand for the unisex satchel has been led by a youthful online fast-fashion clientele.

"Cambridge Satchel is consistently in our top five online accessories sales every week," said Anna Tizard, of Urban Outfitters.

The brand can also claim high-end fashion credibility: last year Commes des Garçons, the venerable fashion-progressive Japanese brand, collaborated with the Cambridge company on a range of bag. The resulting four laptop-sized fluorescent bags promptly sold out at Dover Street market in Mayfair, London.

"A brilliant moment was when top London designer Erdem called us to place an order, and my mother took the call in our kitchen," says Deane. "She was asking him for his surname, and he kept saying: 'It's just Erdem!'"

The success of the bag is down to several factors. There is a spotted-dick Englishness about the Cambridge Satchels, which Deane believes forms part of the appeal. "Our bags are handmade in British factories because there is nothing more British than a satchel," she says.

Her insistence on a "handmade in Britain" label ensures her satchels are imbued with the wistful nostalgia of schooldays, making them as attractive to parents for themselves as for their children.

The current fashion trend for satchels has also fuelled demand. The Céline Classic bag, which carries a £2,000 price tag, has shades of satchel styling, while Mulberry's Alexa bag (£785) is similar in style to the classic schoolbag. By contrast the price of a Cambridge Satchel is a more modest £139, making it a relatively affordable, as well as unexpected, fashion success.

The view from the street

Kiki Davis, 19, student I originally saw this in Urban Outfitters and a boutique in Suffolk, but I bought it directly from The Cambridge Satchel Company's website a year ago. It's a mixture of quirky and practical, for a great price too. It still looks as brand new as the day that I bought it.

Shawn Choudhury, 23, fashion PR

I got this from ASOS four months ago. It's bold and unique but cheaper than a designer bag. I chose this one because of its mixture of classic with bright colours .

Kate McDonough, 34, cinematographer

I got this from ASOS a few months ago. It's practical, roomy and versatile – exactly what I need in my line of work. It's also adaptable for night-time outfits too. It reminds me of taking piano lessons as a child, it has a real feeling of love and nostalgia about it.

Isabella Sumner, 23, visitor services host I bought this two years ago. I had the urge to buy a school-style satchel so I searched online and stumbled upon The Cambridge Satchel Company's website. I decided on this bag as it's so big and sturdy. I had my initials engraved on it too. I'm a bit annoyed they're more ubiquitous now, I liked how mine was special!

Karen Lambourne, 24, marketing co-ordinator

I bought this two weeks ago from Urban Outfitters. I bought this one as soon as I saw it, because I loved that it was a fluorescent update on a classic brown satchel. It's well crafted from decent leather, and for what it is, it's not too pricey either.