It wasn't the first time she had noticed it, but when Kerry Brennan took her two-year-old daughter Frances shopping a little over a year ago, the issue suddenly became a lot more personal. The pair were browsing in the toy aisles of their local branch of Debenhams when the toddler spotted some of her favourite space-themed toys. "She pointed at the sign on the shelf and said: 'Look, that sign says planets!'" says Brennan. "And of course the sign said 'boys'.

"It bothered me that in a year or two, she would start reading and suddenly she would be confronted by this ridiculous message imposed by the retailer that some of the things she loves – like space and dinosaurs and fire engines – are not for her because she is a girl."

Some days later, Brennan logged on to the parenting website Mumsnet to find a discussion under way between other parents who despaired of gendered toy shelves. "So I started another thread: 'Does anybody want to join an effort to convince retailers to stop labelling toys for girls and boys?' There was a lot of interest."

Just over a year later, the small group of concerned parents that grew out of that conversation, now operating under the campaign name Let Toys be Toys, has already had a greater impact on Britain's high streets than any of them could have imagined. Twelve leading retailers – including Toys R Us, The Entertainer, Hobbycraft and Next – have agreed to review the way they display toys after being contacted by the campaigners and their supporters and told that limiting children's choices based on their gender was out of date and unacceptable.

Sainsbury's wrote to say that it agreed with the direction of the campaign and would be phasing out signage that classed toys according to gender. Marks & Spencer said it would change the labelling of some toys badged "boy's stuff" and "Li'l Miss Arty".

Boots said it was "dismayed that our attempts to help customers shop in our store hasn't worked in the way we wanted it to", after acknowledging that its decision to display toys from the Science Museum in London under a sign reading "boys' toys" was wrong. (A mini tea set, Fuzzy Felt princess set and Hello Kitty craft kit were under "girls' toys".) "We've always been proud of supporting women in science and, in particular, in their careers in pharmacy," the retailer said in a letter. "It was never our intention to stereotype certain toys."

Following a survey by its members, the group claims that a year ago, half of the shops it assessed used gender labels on toy shelves, compared with a fifth in 2013.

Megan Perryman had never been involved in a campaign until she, too, signed up as a result of the online thread. She thinks its success is because the group decided early on to be focused in its goals. "It's such a huge issue when you're talking about gender equality in children. So we decided fairly early on that we needed to have a very specific ask." For the first year of the campaign, they agreed they would ask retailers to display toys by theme, not gender.

This is not the first attempt to challenge sexist labelling – Hamleys bowed to pressure in 2011 to remove signs that labelled entire floors of its flagship London store by gender – but no previous campaign has achieved such an impact.

Certainly, Let Toys be Toys has provoked a storm of interest within the industry, according to Billy Langsworthy, deputy editor of the trade magazine Toy News. A story on its website noting Toys R Us's decision to stop displaying toys by gender was shared online more than 7,000 times – a popular article would normally attract 15 or 16 shares, says Langsworthy. "The striking thing about this campaign is the speed, that it's all taken off so successfully," he says. "The Toys R Us decision was a really big moment for them in terms of profile. It made other retailers look towards them and think, maybe this is something we should look at."

Mike Coogan, marketing and e-commerce director at Toys R Us, said: "We understand that children have many diverse interests, and we consistently strive to portray that in our aisles and in our advertising materials. We are always keen to listen to our customers, and in this respect it is good to have an ongoing dialogue."

A spokesperson for Next said its toy range, some of which was labelled "boys' stuff", had this year been branded under one unisex title, The Little Gift Co.

"Our packaging designs, labelling and in-store signage was intended to help customers choose appropriate gifts last year; however, we realise that these classifications could be misleading," said the spokesperson.

The campaigners say their battle is far from won, — they stress the importance of holding retailers to their commitments, and intend to take the argument to toy manufacturers and book publishers next year. So why do they think that, in the space of 12 months, they have had such an impact?

"I think it's partly that we tapped into something that people were thinking anyway," says Perryman. "But it's also because we're being reasonable. We are not asking them to promise the world, we understand that they are driven by profit, we understand that they can't change everything overnight.

"But asking people to say 'That's a doll and that's a car', as opposed to 'That's for girls and that's for boys', is a reasonable ask."