Harper Lee may be at number one in the Amazon best-sellers list with Go Set a Watchman – but there are currently nine adult colouring books also in the top 20.

In fact, colouring book manufacturers said they are struggling to keep up with demand as thousands of grown-ups seek stress relief and mindfulness in the form of reverting to a childhood activity – in an adult way of course.

In the top 20, Millie Marotta’s Animal Kingdom – A Colouring Book Adventure – is currently at number five, with nearly 200 days in the top 100. There’s also a colouring book for people who can’t sleep, one focused on mindfulness, another two for therapy, then an enchanted forest theme, a secret garden and tattoo designs for big people to colour in.

Amy Yodanis, head of marketing at Quatro – which will have 1.3 million colouring books in print this year, said: “We cannot print them fast enough.

“We are getting orders of 60,000 at one time from some of our biggest retailers.”

On social media, colouring clubs and competitions have been set up, with Dover Publications planning a National Colouring Day in the US on August 2. Make it international, please!

Colouring or art therapy apparently reduces stress and anxiety and provides a great way to calm down and unwind at the end of the day.