The Blue Valentine actress, 43, will voice the Toxic singer’s much-anticipated book, while Britney, 41, herself will record an introduction.
Announcing Michelle’s role, the Womanizer singer said in a statement: ‘This book has been a labor of love and all the emotions that come with it.
‘Reliving everything has been exciting, heart-wrenching, and emotional, to say the least. For those reasons, I will only be reading a small part of my audiobook.’
She added to People: ‘I am so grateful to the amazing Michelle Williams for reading the rest of it.’
The Shutter Island star, who had been nominated for five Oscars, said to the publication: ‘I stand with Britney.’
It had been previously reported that Britney had hired a celebrity to narrate her memoir, with it claimed she had first approached Reese Witherspoon, who was unavailable.
The music icon confirmed her book last year, amid reports that she had signed a deal reported to be worth $15 million (£12.3m).
Fans were later given a glimpse into the memoir in July, with the cover art and title unveiled ahead of the release.
Since then, Britney has faced more turmoil in her private life after husband Sam Asghari filed for divorce in August, bringing their 14-month marriage to an end.
However, sources claimed that she gave her ‘final sign-off’ on the book weeks before their split, meaning there will not be ‘any changes made’ to reflect their break-up.
While little else is known about what will make it into those final pages, Britney has previously said she had ‘lots of therapy’ to complete the book.
She previously teased on Instagram: ‘I worked my ass off for this book, I had a lot of therapy to get this book done, so you guys better like it.
‘And if you don’t, that’s cool too.’
The Woman In Me was described as a ‘brave and astonishingly moving story about freedom, fame, motherhood, survival, faith, and hope.’
It will ‘illuminate the enduring power of music and love – and the importance of a woman telling her own story, on her own terms.’
The Woman In Me will be published by Gallery Books, an imprint of Simon & Schuster, and is available to pre order now.
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