Prince has said it would have been an “impossibility” for him to have sold out 21 shows in London earlier in his career.

The pop star, who turned 50 earlier this year, has released a book about the residency, which took place at the O2 Arena last summer.

Speaking at its launch, Prince told the Canadian Press: "I couldn't have sold out 21 nights in London in the peak of my career; it would have been an impossibility.”

In a philosophical interview, the singer said he was looking forward to his twilight years “where everything is just open sky”.

He added: "I wish this for every artist: freedom."

21 Nights features the singer’s poetry, as well as exclusive backstage shots captured by photographer Randee St. Nicholas.

The book is also accompanied by a live CD comprised of new and old songs from Prince’s after show parties at the Indigo2.

(by Jason Gregory)