Copies of Harper Lee's eagerly awaited novel Go Set a Watchman are now on sale in UK bookshops, where it was released at midnight.

The book is set 20 years after the events of Lee's 1960 Pulitzer Prize-winning novel To Kill a Mockingbird, but was actually written beforehand.

Watchman follows Scout, now known as 26-year-old Jean Louise, as she returns to her Alabama hometown from New York.

It is the most pre-ordered book title on Amazon since the final Harry Potter.

Publicity-shy author Lee, who is now 89 and living in a nursing home in Monroeville, Alabama, originally wrote the book in 1957, before reworking it with her editor to become courtroom drama To Kill a Mockingbird.

The story of racism and injustice in the fictional town of Maycomb in the American South went on to sell 40 million copies and be studied in schools around the world.

Mockingbird was also made into an Oscar-winning film starring Gregory Peck as lawyer Atticus Finch, who defends an innocent black man accused of raping a white woman.

The existence of Go Set a Watchman was revealed in February and it is being released in 70 countries simultaneously.

The opening chapter of the novel was published for the first time on Friday, and many early reviews revealed that in later years Finch had in fact become "a bigot".

"This story is of the toppling of idols," wrote Sam Sacks in the Wall Street Journal, adding that it was "a distressing book, one that delivers a startling rebuttal to the shining idealism of To Kill a Mockingbird".

Sacks said: "For the millions who hold that novel dear, Go Set a Watchman will be a test of their tolerance and capacity for forgiveness."

The New York Times said the revelation could "reshape  Lee's legacy" and made for "disturbing reading".

Writing for The Guardian, Mark Lawson said: "If the text now published had been the one released in 1960, it would almost certainly not have achieved the same greatness.

"This is not so much due to literary inferiority, but because Go Set a Watchman is a much less likeable and school-teachable book."

Lawson added that it was "in most respects, a new work, and a pleasure, revelation and genuine literary event... This publication intensifies the regret that Harper Lee published so little."

However, The Telegraph gave Go Set a Watchman a two-star review, with Gaby Wood writing "Harper Lee's editor deserves a Pulitzer for turning this ghostly first draft into the masterful To Kill a Mockingbird".

She added: "It feels like a sequel. But really, it's more like a ghost: the spectre of Lee's restless, ardent thoughts in progress."

The Independent's Arifa Akbar said: "We will never be able to read Mockingbird in the same way again, and never see Atticus in the same light again.

"Despite the boldness and bravery of its politics, Go Set a Watchman is a very rough diamond in literary terms."

She added: "Whatever its failings [it] can't be dismissed as literary scraps from Lee's' imagination. It has too much integrity for that."