Go Set A Watchman, which was written beforehand by author Harper Lee, was, until now, thought to have been lost.

The 304-page book will be published by Harper Collins. The terms of the deal have not been disclosed.

The Pulitzer-winning To Kill a Mockingbird was published in 1960 and has sold more than 40 million copies worldwide.

Set in Maycomb, Alabama, during the Great Depression, it is the story of a young girl whose father, a lawyer, is appointed to defend a black man accused of raping a white woman.

In 1962, it was adapted into a film starring Gregory Peck as attorney Atticus Finch. The role won an Oscar for Peck.

Harper Collins is planning a first printing of 2 million copies of Go Set A Watchman.

Lee said she had originally written Go Set A Watchman but, after advice from her editors, who were more interested in the flashbacks to the character Scout's youth, was persuaded to write another book, which became To Kill A Mockingbird.

Scout, also known as Jean Louise Finch, is the book's narrator, looking back on the events of her youth as an older woman.

"I hadn't realised it had survived," Lee said in a statement issued by Harper Collins. "So was surprised and delighted when my dear friend and lawyer Tonja Carter discovered it.

"After much thought and hesitation I shared it with a handful of people I trust and was pleased to hear that they considered it worthy of publication. I am humbled and amazed that this will now be published after all these years."

Go Set A Watchman is also set in Maycomb, roughly 20 years after the events of To Kill A Mockingbird.

A summary from the publisher says: "Scout has returned to Maycomb from New York to visit her father, Atticus. She is forced to grapple with issues both personal and political as she tries to understand her father's attitude toward society, and her own feelings about the place where she was born and spent her childhood."

Publisher Jonathan Burnham said: "This is a remarkable literary event. The existence of Go Set a Watchman was unknown until recently, and its discovery is an extraordinary gift to the many readers and fans of To Kill a Mockingbird.

"Reading in many ways like a sequel to Harper Lee's classic novel, it is a compelling and ultimately moving narrative about a father and a daughter's relationship, and the life of a small Alabama town living through the racial tensions of the 1950s."

The book will be released in July.