PARIS — When Otto Frank first published his daughter’s red-checked diary and notebooks, he wrote a prologue assuring readers that the book mostly contained her words, written while hiding from the Nazis in a secret annex of a factory in Amsterdam.

But now the Swiss foundation that holds the copyright to “The Diary of Anne Frank” is alerting publishers that her father is not only the editor but also legally the co-author of the celebrated book.

The move has a practical effect: It extends the copyright from Jan. 1, when it is set to expire in most of Europe, to the end of 2050. Copyrights in Europe generally end 70 years after an author’s death. Anne Frank died 70 years ago at Bergen-Belsen, a concentration camp, and Otto Frank died in 1980. Extending the copyright would block others from being able to publish the book without paying royalties or receiving permission.

In the United States, the diary’s copyright will still end in 2047, 95 years after the first publication of the book in 1952.