The chain said the collection of the 16-year-old climate activist's speeches had an "urgent message".

Waterstones normally chooses a book of the year, but this year has added the accolade for author of the year.

It decided the best book of 2019 was Charlie Mackesy's The Boy, the Mole, the Fox and the Horse.

Charlie Mackesy created his book after his illustrations were widely shared online

The former Spectator cartoonist's book combines his illustrations with a hand-written story of friendship between a boy and three animals.

"The Boy, the Mole, the Fox and the Horse is a wise and beautiful tale calling for love, friendship and courage," said Waterstones' Kate McHale.

"Greta Thunberg's speeches have been an inspiration to millions... Both choices are testament to the extraordinary power of books to move and shape us."

Waterstones said it had added the author prize this year to reflect "how strongly the titles resonated with our booksellers and customers".

Thunberg has not commented on the accolade. She recently turned down an environmental award and prize money, saying "the climate movement does not need any more awards".

The Waterstones book of the year shortlist:

• The Testaments by Margaret Atwood
• Queenie by Candice Carty-Williams
• The Binding by Bridget Collins
• Underland by Robert Macfarlane
• The Boy, the Mole, the Fox and the Horse by Charlie Mackesy
• Prisoners Of Geography: Our World Explained in 12 Simple Maps by Tim Marshall, illustrated by Grace Easton and Jessica Smith
• The House Without Windows by Barbara Newhall-Follett, illustrated by Jackie Morris
• Lanny by Max Porter
• On The Origin Of Species by Sabina Radeva
• Dishoom by Shamil Thakrar, Kavi Thakrar & Naved Nasir
• No One Is Too Small To Make A Difference by Greta Thunberg

The 2018 book of the year was Normal People by Sally Rooney, with Philip Pullman winning at the end of 2017 for La Belle Sauvage.