My hospital patients have no idea I write Hollywood blockbusters on my days off

She is one of America's bestselling authors with a host of celebrity fans such as Victoria Beckham and Michelle Keegan, both of whom have been spotted reading her novels.

Now Freida McFadden's worldwide hit The Housemaid is being made into a blockbuster film starring The White Lotus star Sydney Sweeney and Mamma Mia!'s Amanda Seyfried.

But despite being a hugely successful writer, the brain injury doctor based in Boston has no intention of giving up the day job entirely...

She said: "Being a doctor is my job and I don't want to lose my skills, although I only work one day a week now.

"I don't want to stop being a doctor. It is important to me but I prefer writing if I'm honest - but then most people prefer doing their hobby to their job.

"No one at the hospital really knows who I am, not most of the staff and certainly not my patients. Freida McFadden is just my pen name, not my doctor name.

"At the beginning, writing was just a hobby, a way to relax, like some people play golf. Not that I didn't want to get published but I had a few knock-backs at college. Then when self-publishing became a thing, I thought 'why not?'."

It is clear the Harvard-educated medic's background as a doctor influenced some of her early books like The Devil Wears Scrubs and Brain Damage.

Since 2013 she has published 24 novels - with the 25th, The Crash, due out next year.

The Housemaid has sold 1.8 million copies worldwide, with a record-breaking half-a-billion page reads in Amazon's Kindle Unlimited subscription programme, and has been on the New York Times bestseller list for more than a year. It is the first of her novels to be made into a movie.

It will be directed by Bridesmaids film-maker Paul Feig, with filming due to start next year.

Freida, 57, a married mother of two teen agers, said: "I am so excited about the film. It is a dream come true for any author to see their book being made into a film.

"Not that I have been involved in the casting or anything but I am thrilled about Sydney Sweeney playing Millie."

The Housemaid is the story of ex-con Millie, - Sweeney, 27 - who having been released from prison is living in her car until she manages to secure a job as a live-in housemaid with a sinister wealthy woman, Nina, to be portrayed by Seyfried, 38.

But after starting the job she realises Nina's seemingly perfect life with her good-looking and successful husband and young daughter is not at all that it seems.

The book is the first in a hugely successful trilogy completed by The Housemaid's Secret and The Housemaid is Watching.

Frieda is famed for her page-turning crime thrillers and she herself is a huge fan of fellow American crime writer Tess Gerritsen, who is best known for The Surgeon.

"I love her books. She is a big influence. I just love the crime thriller genre," she said.

"I have tried writing other things. Early on in my writing career I wrote what would have been called women's fiction but if I changed now people would just ask, 'why isn't anyone being murdered?'."

The Housemaid by Freida McFadden (Little, Brown, £9.99) is out now. Visit www.expressbook shop.com or call Express Bookshop on 020 3176 3832. Free UK P&P on online orders over £25.