Sir Paul McCartney has penned a picture book for children.

The 76-year-old music icon has announced he is releasing the book 'Hey Grandude' which will tell the story of a ''magical'' gentleman called Grandude and the fun he has with his four grandkids.

Paul got the idea for the story when one of his eight grandchildren referred to him by the name.

Explaining the title, the former Beatle said: ''I've got eight grandchildren and they're all beautiful and one day one of them said to me 'Hey Grandude!' I said 'What?' and I thought, 'I kind of like that,' so from then on I was kind of known as Grandude.

''So I thought, you know what, it's actually a nice idea for a book, so I started to write some stories, and then I talked to the book publishers and they liked what I was doing.''

Paul hopes the tome will resonate with grandparents everywhere and become a book that they read to their grandchildren.

He said: ''He calls them (his grandchildren) 'Chillers'. They love him and they go on adventures with him and he's kind of magical, so you'll see that in the book. I wanted to write it for grandparents everywhere - and the kids - so it gives you something to read to the grandkids at bedtime.''

The book is to be illustrated by Kathryn Durst, who is absolutely delighted to be teaming up with Paul.

She said: ''I have wonderful childhood memories of my dad taking my siblings and me on exciting adventures, just like Grandude does in the book. I am very excited to be able to work with Paul McCartney on this project. It has been such a treat to collaborate with such an inspiring and accomplished artist. We both want to make an exciting and beautiful book that kids will want to read again and again!''

'Hey Grandude' will be published in September 2019.