The previously completed merger of social media giant Facebook and Giphy, the largest provider of GIFs, and meme sharing services, has run into a potential roadblock from the U.K.’s Competition and Markets Authority (CMA).

Following an investigation, the CMA has provisionally found that Facebook’s 2020 takeover of Giphy would negatively impact competition between social media platforms.

The deal was announced in May last year and is valued at $400 million, Variety understands.

If the CMA competition concerns are ultimately confirmed, it could require Facebook to unwind the deal and sell off Giphy in its entirety. Retrospective examinations of mergers and takeovers are rare, but not unheard of in the U.K.

The CMA provisionally found that Facebook’s ownership of Giphy could lead it to deny other platforms access to its GIFs. The body also notes that Facebook could change terms of access.

“For example, Facebook could require Giphy customers, such as TikTok, Twitter and Snapchat, to provide more user data in order to access Giphy GIFs. Such actions could increase Facebook’s market power, which is already significant,” the CMA said in a statement. The CMA’s analysis records that Facebook’s platforms — Facebook, WhatsApp, and Instagram — account for over 70% of the time people spend on social media and are accessed at least once a month by 80% of all internet users.

Stuart McIntosh, chair of the independent inquiry group carrying out the investigation, said: “Millions of people share GIFs every day with friends, family and colleagues, and this number continues to grow. Giphy’s takeover could see Facebook withdrawing GIFs from competing platforms or requiring more user data in order to access them. It also removes a potential challenger to Facebook in the £5.5 billion [$7.6 billion] display advertising market. None of this would be good news for customers.”

“While our investigation has shown serious competition concerns, these are provisional,” McIntosh added. “We will now consult on our findings before completing our review. Should we conclude that the merger is detrimental to the market and social media users, we will take the necessary actions to make sure people are protected.”

“We disagree with the CMA’s preliminary findings, which we do not believe to be supported by the evidence,” a Facebook spokesperson told Variety. “As we have demonstrated, this merger is in the best interest of people and businesses in the U.K. – and around the world – who use Giphy and our services. We will continue to work with the CMA to address the misconception that the deal harms competition.”

Giphy currently has no employees, revenues or assets in the U.K., meaning that the CMA has no jurisdiction over the deal, Facebook argued in its submission to the investigation. Facebook also argued that Giphy’s paid alignment services do not constitute display advertising under the CMA’s own market definition.

Facebook also contends that if Giphy could be considered a potential competitor in advertising, then, by the same logic, thousands of other companies would have to be considered actual or potential competitors to Facebook as well.

The merger is also being reviewed by other competition authorities in the U.K. These are in touch with the CMA.

The CMA is now open to responses from interested parties to its provisional findings by Sept. 2 and its notice of possible remedies by Aug. 25. These will be considered before the CMA issues its final report, due by Oct. 6.