Cats in Europe might have to be kept on leashes, if two environmental lawyers get their way.
Lawyers in the Netherlands earlier this week hinted at legal action against the Dutch government, saying it's doing nothing to protect birds.
"Even if it is not your intention to harm wild animals when you leave the cat flap open, that is what happens on a large scale," Han Somsen told The Telegraph.
"Whoever starts a lawsuit against the Dutch government, who must enforce European rules, has a good chance of success," said his partner Arie Trouwborst.
They say letting cats roam free conflicts with the EU's Birds and Habitats Directive, which protects not just birds but their eggs and habitats.
Domestic cats "impact biodiversity through predation, disturbance, competition, disease and hybridization" to an extent worse than most people would assume, they wrote in a paper published in the Journal of Environmental Law.
The EU however says pets and animals have just as much freedom of movement as people.
"The [European Commission] is a strong defender of free movement rights - including of cats," environment spokesman Enrico Brivo told The Telegraph.
"We categorically deny that the commission will oblige cats to be held on a leash at all times."
He said cats are "not among the main pressures and threats to biodiversity".
Dogs, cats and ferrets are able to travel freely around the EU, The Telegraph reports. British pets will lose that right if Brexit happens.