Germans will no longer be able to buy erotic eBooks between 6am and 10pm under new rules.
If book sellers are found to be flouting the law, they could face fines of up to 50,000 euros (£35,000).
The new mandate is an extension of a previous law that applied to German cinemas showing adult films, which was originally created in 2002.
However, this law has now also been extended to cover sales of digital books.
The extension follows a legal complaint over an adult-themed book called Schlauchgeluste (translation: Pantyhose Cravings), which argued that the title was too easy to obtain for children.
According to the German Publishers and Booksellers Association (GPBA), retailers are currently trying to determine how best to enforce the new rules.
“A time lock doesn’t seem to make a lot of sense to us as kids can find ways to circumvent that,” explained Jessica Sanger, a member of the GPBA legal team.
Sanger added: “A system that requires proof of age is harder to circumvent and that is the way we see things going.”
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According to a report on The Independent, book sellers in Germany have yet to shut down adult eBook sales in the day.
It’s also not yet clear how companies will vet books to determine whether they fall within the adult-themed category.
The move has not gone without criticism, however, as prominent digital media blogger Nate Hoffelder explains.
“Given the prevalence of adult content on websites outside of Germany, trying to control access on sites in Germany is just nuts,” wrote Hoffelder.
He continued: “Even King Canute knew that he could not hold back the tide, but apparently German regulators lack that level of common sense.”