Author Harper Lee has said she is "happy as hell" about the publication of a sequel to her bestseller To Kill A Mockingbird after concerns were raised about the extent of her involvement in the project.

The reclusive 88-year-old suffered a stroke in 2007 and was reported to have gone blind and The Washington Post reported "questions have been raised about how much control Lee is exercising" over the new book.

The novel, called Go Set A Watchman, was written before To Kill A Mockingbird but was rejected by publishers who set her to work on the novel that made her famous.

To Kill A Mockingbird, set around a rape trial in the racially-divided Deep South of the US, has sold more than 40 million copies since it was published in 1960.

Its central characters, Scout, her brother Jem and their lawyer father Atticus, were brought to life in a 1962 film starring Gregory Peck.

The new book revolves around the now-adult Scout's return to her native Alabama from New York to visit her father.

A spokeswoman for her publisher said: "Harper Lee still enjoys reading and uses a magnifying machine from the New York Institute for the Blind to read books, newspapers and documents."

Lee herself said: "I'm alive and kicking and happy as hell with the reactions to Watchman."

She previously said she had not realised the manuscript of her new book had survived and said she was "humbled and amazed" it was going to be published now.

Go Set A Watchman will be published on July 14 by William Heinemann, which was the original UK publisher of To Kill A Mockingbird.