Rudyard Kipling might have plagiarised his most famous literary work "The Jungle Book" from an unknown source. A recently discovered signed letter written in 1985 reveals that the celebrated English author admitted to stealing the idea for the book.

The one-page handwritten letter is being sold by Andruiser Autographs for £2,500 [approximately AUD 4,000] at an auction.

It is addressed to an unidentified woman, and reads:

"Dear Madam:

"I have been absent from home for some days. Hence the delay in answering yours of no date, in regard to my account of the Law of the Jungle.

"I am afraid that all that code in its outlines has been manufactured to meet 'the necessities of the case': though a little of it is bodily taken from (Southern) Esquimaux rules for the division of spoils. In fact, it is extremely possible that I have helped myself promiscuously but at present cannot remember from whose stories I have stolen."

He closed the letter with, "Very sincerely, Rudyard Kipling."

According to the Telegraph, Andrusier bought the letter from another dealer, Jarndyce Antiquarian Booksellers, in May when it was listed in the New York Book fair. It was found in April among a collection of uncategorised books and letters.

"The Jungle Book," published in 1894, is a collection of stories, with a young jungle boy named Mowgli as the main character. It is one of the most loved English language children's books of all time. "Law of the Jungle" is a term Kipling used to describe the law used by wolves and other animals in the jungles of India.

The first edition of the book, discovered at the National Trust's Wimpole Hall in 2010, revealed that it was dedicated to his daughter Josephine, who died at age six in 1899.