Three million copies of Johanna Basford’s adult colouring book Secret Garden have been sold in China in less than three months, its publisher has announced, dubbing Beijing the “adult colouring-in capital of the world”.

As the Beijing international book fair takes place this week, Laurence King Publishing said that Secret Garden had sold 6.8m copies around the world, with 3m of these sales in China, where it was published on 1 June. The title has also been a major hit in Brazil, where it has sold just over 1.1m copies, said Laurence King, with 650,421 copies sold in the US, half a million in South Korea, and 477,658 in the UK to date.

Basford’s collection of pen-and-ink illustrations of a garden “sheltering all kinds of tiny creatures just waiting to be found”, first published in 2013, has been one of the frontrunners in the new fad for adult colouring books. Describing the craze as a “global phenomenon”, Laurence King said that Basford’s second title, Enchanted Forest, had sold more than 2m copies worldwide since it was published in March.

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There are currently four adult colouring titles in Amazon.co.uk’s top 10, with the online retailer revealing on Tuesday that Millie Marotta’s Animal Kingdom is its second bestselling title of the year to date, behind cookbook Deliciously Ella. Three more colouring books are included in Amazon’s top 20 book titles of the year so far - The Art Therapy Colouring Book, The Mindfulness Colouring Book and Secret Garden. Brits “appear to have adopted colouring as a form of relaxation,” according to Amazon.

“I think it is really relaxing, to do something analogue, to unplug,” Basford said in April. “And it’s creative. For many people a blank sheet is very daunting – with a colouring book you just need to bring the colour. Also there’s a bit of nostalgia there. So many people have said to me that they used to do secret colouring in when their kids were in bed. Now it is socially acceptable, it’s a category of its own. These are books for adults. The art in my books is super-intricate.”