Facebook could have to sell Giphy, a search engine for Gifs, which it bought for a reported $400m (£289m) last year.
The Competition and Markets Authority provisionally found Facebook owning Giphy "could lead it to deny other platforms access to its Gifs".
The CMA will now consult before making a final conclusion. And if its concerns are confirmed, the it may require Facebook to sell Giphy.
Facebook said the findings were "not supported by the evidence".
Giphy's vast library of looping short video animations is hugely popular - including among Facebook's competitors.
Gif stands for Graphics Interchange Format, an image format developed in the 1980s to display static and moving images.
They have become a staple of social-media posts and comments.