The Sun said two unbound copies of Rowling's latest offering, "Harry Potter and the Order of the Phoenix," were discovered in eastern England close to Clays Ltd., a firm contracted to print hundreds of thousands of copies.
The 40-year-old man who found them handed over the books to The Sun, the newspaper said.
Publisher Bloomsbury PLC said it appeared the books had been stolen. Suffolk Police were investigating.
"The matter is currently under investigation, with the suspicion that theft is involved," the publisher said in a statement.
The highly anticipated fifth installment of the Harry Potter series hits bookstores on June 21. Fans have waited nearly three years for the schoolboy wizard's fifth adventure.
Within hours of the announcement in January that "Order of the Phoenix" was coming out, the book topped the best-seller lists of Amazon.com and Barnes & Noble.com.
The plot has been kept under tight wraps. Hunger for news about Potter No. 5 drove an American collector to pay more than $45,000 recently for a card full of clues about the plot. It contained 93 words, including "Ron," "broom" and "sacked."
The Sun said the books were stashed in a safe at its London headquarters in London while the newspaper made arrangements to return them to Bloomsbury.
Rowling's four previous Potter novels had worldwide sales of more than 190 million. "Harry Potter and the Order of the Phoenix" is 768 pages long, and by word count one-third longer than its predecessor, "Harry Potter and the Goblet of Fire."