Erotic ebooks like EL James's Grey cannot be sold in Germany before 10pm, new rules state, and sales must be stopped at 6am every morning.
The rules have applied to German cinemas showing erotic films since 2002, in an attempt to stop children from seeing inappropriate adult content. But now they have been extended to include ebooks, which can be downloaded in a few seconds from online stores like Amazon.
Booksellers found flouting the new rules could find themselves receiving legal action by the government's Federal Department for Media Harmful to Young Persons. The change in how the law is applied was caused by an erotic book called Pantyhose Cravings, which was the subject of a legal case over how easily such novels can be purchased.
A fine of €50,000 (£35,000) could be handed down to any retailer found selling erotic ebooks between 6am and 9.59pm, but it is more likely that they will receive a warning at first, given how recently the law has been extended.
According to the BBC, it has been suggested that booksellers will have to keep tabs on ebooks which count as "youth endangering", put them in a specific section of the website, and make this section invisible during the daytime.
But this could be difficult to enforce. Jessica Sanger, from the legal team of the German Publishers & Booksellers Association, said: "A time lock doesn't seem to make a lot of sense to us as kids can find ways to circumvent that. A system that requires proof of age is harder to circumvent and that is the way we see things going."
Online stores could instead check the age of buyers by looking at their credit card information, but this would mean erotic titles could be sold at any time of day.
Digital media blogger Nate Hoffelder said on his website: "Given the prevalence on websites outside of Germany, trying to control access on sites in Germany alone is just nuts. Even King Canute knew that he could not hold back the tide, but apparently German regulators lack that level of common sense."