Macmillan

Publisher Macmillan Children's Books billed the new book as "a fresh and exciting adventure that has all the hallmarks of a Julia Donaldson and Axel Scheffler classic".

The first two books have sold 18 million copies, Macmillan said.

Lucy, from Hull, will run her first marathon on Sunday alongside her dad to raise money for Macmillan Cancer Charity in memory of her mother, Clare, who died from cancer when the teenager was nine years old.

On race day, Lucy will be wearing a Macmillan top with the letters 'MUM' on the back.

Hart's publisher, Pan Macmillan, declined to comment, while Sunak has been approached for comment.