It started with a tickle. Well, Mr Tickle, to be precise.

When seven-year-old Adam Hargreaves asked his dad Roger what a tickle looked like, he started a phenomenon that has lasted more than three decades and entertained millions of children across the globe.

Roger Hargreaves, an advertising copywriter who yearned to be a cartoonist, sat at the breakfast table and drew a little orange person with unfeasibly long arms, a big toothy grin and a blue hat. And so Mr Tickle was born.

The series celebrates its 35th anniversary this year and there are now more than 80 Mr Men and Little Miss characters and more than 100 million books have been sold worldwide.

To mark the milestone anniversary, two new books - Mr Birthday and Little Miss Birthday - are being launched today.

A major month-long exhibition is also being held at the Animation Art Gallery in London.

After making their fortune, the family moved to Sussex. In 1988, Adam, then 24, taught himself to draw and took over the series when his father suffered a series of strokes and died suddenly, aged 53.

He introduced some new friends including Mr Rude, Mr Cool, Mr Good, Little Miss Bad, Little Miss Scary and Little Miss Whoops.

Adam, who lives in Cross-in-Hand, near Heathfield, said: "I've got a very different style of drawing to my dad so it took me a while to master them, simple as they are.

"I wasn't wholly happy with what I was drawing until about five years after my father's death. To begin with it was a bit forced.

"But I did enjoy it. Because I was ignorant of the whole business, I never felt too much in my dad's shadow. Drawing Mr Men is a lovely job. To begin with I did think of my dad but it became part of my life."

He said the success of the books lies in their simplicity.

He said: "They are about us, about human emotions and characteristics so everybody can feel something for the characters.

There's an empathy, even if it's Mr Grumpy or Mr Uppity. They are a little bit of each of us and that's why they don't date in the same way some other children's books do.

"I'm sure he based his characters on real people but I couldn't tell you who. There were bits and pieces from everybody he met. Each of the characters is a stereotype and some of the more complicated emotions he was tackling I'm sure came from friends and colleagues."

Two years ago Adam became Mr Millionaire when he sold the business for a reported £28 million to Chorion, best-known for owning the rights to big-name children's titles such as Enid Blyton's Noddy.

He remains a part-time creative consultant and is pursuing a fine art career, concentrating on landscape oil paintings.