Sarah Duchess of York said the message she wants readers to take away from her new Mills & Boon book is to “fight for what you believe is your truth”.

The Duke of York’s ex-wife described the book – Her Heart For A Compass – as her “sweeping, fabulous, historical novel” set in the 1870s and said her heroine Lady Margaret is “a very rebellious lady”.

The book, which draws on Sarah’s own life journey and incorporates research into her ancestry, is a fictional account of the life of the duchess’s great-great-aunt Lady Margaret Montagu Douglas-Scott.

Lady Margaret is banished from polite society after fleeing an arranged marriage.

In a dramatic video trailer for the novel, the duchess said: “Lady Margaret defies the echelons of high society.

“She follows her heart. She is in search of what she believes is the truth of her own heart.”

In the trailer, which features clips of the duchess in period costume looking intensely at the camera and into the distance, Sarah asks: “Can you take yourself to Victorian times? And can you move into a place with the courage and the bravery of Lady Margaret and how she manages to do that?

“So come on this journey with me from the drawing rooms of Windsor, to the country houses of Scotland and Ireland, and then to the slums of London, and on, on across the seas to the hustle and bustle of New York City in 1870. It’s so exciting. Come, join us.”

Sarah said she is “so deeply proud” of the book, adding: “It just moves me to even share it with you and to talk to you about it.”

Talking about her heroine, the duchess said: “There are some obvious parallels between me and my heroine, my great-great aunt, Lady Margaret Montagu Douglas-Scott: she’s a redhead; she is determined to make her own way in life; and like me she can be headstrong.

“She also takes up a career in writing, and works really hard for the charitable causes she cares so passionately about. But ultimately Her Heart For A Compass is a work of fiction, not an autobiography.”

She said she thinks the challenges Margaret faces will resonate with readers.

“The main message I would like readers to take away from Margaret’s story is that no matter whether you make mistakes or get it all wrong, you keep going and fight for what you believe is your truth.

“You have to lead with your heart. Personally, I feel more robust, authentic, confident in myself and who I am and where I’m going than ever, and that feeling is very much reflected at the end of Margaret’s journey in Her Heart For A Compass,” Sarah said.

The duchess co-wrote the book with Marguerite Kaye, who has written more than 50 novels for Mills & Boon set in a variety of historical eras.

Ms Kaye said she and Sarah have become close friends, adding: “Lady Margaret’s story is the biggest book I’ve ever written, both in scope and wordcount.

“It is double the length of a Mills & Boon Historical Romance so there’s more space to include abundant amounts of research to provide colour and historical context to Lady Margaret’s story, an aspect of writing which the duchess and I both loved, since we share a real passion for history.”

Sarah said she is “so excited to be diving into” a second novel for Harper Collins and Mills & Boon, again with Ms Kaye as her co-author.

“I really feel this book marks a new phase of my life and a whole new chapter for me.

“I’m ready to embrace the opportunities and challenges of writing adult fiction, and to build on what I have learned when writing Her Heart For A Compass.

“I can’t reveal too much about the new book at this point, but it will be another period drama, and will build on the world which I have created for this first book,” the duchess said.

The duchess fell in love with Prince Andrew and brought renewed vigour to the monarchy in the 1980s alongside Diana, Princess of Wales, but struggled to come to terms with the disciplines of royal life.

Born Sarah Ferguson and nicknamed Fergie by the press, she once confessed: “I was hopeless from the start… they could never make me the perfect princess.”

She was cast out from the royals amid her “toe-sucking” scandal in the 1990s and fell deeply into debt, owing millions.

The Queen’s former daughter-in-law, who has previously written her memoirs, is the author of children’s books including the Little Red, and Budgie the Little Helicopter series, and was an executive producer of the 2009 historical film The Young Victoria.

She has been reading children’s books throughout the pandemic on her YouTube channel, Storytime With Fergie And Friends.

– Her Heart For A Compass published by Mills & Boon, HarperCollins Publishers, is out on August 3.