The US Supreme Court has upheld a divest-or-ban law targeting TikTok, leaving the video app potentially facing a blackout for its 170mn US users and putting its fate in the hands of president-elect Donald Trump.

The law compels TikTok’s Chinese parent ByteDance to sell the platform by January 19 — the day before Trump returns as US president — or face a nationwide ban.

“There is no doubt that, for more than 170 million Americans, TikTok offers a distinctive and expansive outlet for expression, means of engagement, and source of community,” the Supreme Court wrote in a unanimous opinion published on Friday.

“But Congress has determined that divestiture is necessary to address its well-supported national security concerns regarding TikTok’s data collection practices and relationship with a foreign adversary,” it added.

The decision by the country’s top court on Friday means the video app could “go dark” in one of its biggest markets. But the Biden administration said following the ruling that the outgoing president would not enforce the ban during his remaining days in office.

“Given the sheer fact of timing, this administration recognises that actions to implement the law simply must fall to the next administration, which takes office on Monday,” the White House said.

“TikTok should remain available to Americans,” it added, “but simply under American ownership or other ownership that addresses the national security concerns identified by Congress in developing this law.”

Trump said in a post on Truth Social after the ruling that his “decision on TikTok will be made in the not too distant future, but I must have time to review the situation”, adding that the court’s decision was “expected, and everyone must respect it”.

In a video posted on TikTok following the decision, the group’s chief executive Shou Zi Chew gave no reassurances on whether the app would continue to function in the US on Sunday, but lavished Trump with praise.

“I want to thank President Trump for his commitment to work with us to find a solution that keeps TikTok available in the United States. This is a strong stand for the First Amendment and against arbitrary censorship,” he said, adding that the president-elect “truly understands” the platform.

Unless a buyer is found for TikTok within days, it will be unlawful for companies to provide services to distribute or host the video app, or they will face fines of $5,000 per user.

It is unclear whether tech groups such as Apple and Google will continue to make the app available on their app stores, or whether Oracle, which provides cloud and data services to TikTok in the US, will continue to do so from Sunday. It is also unclear whether the company could also take itself offline deliberately, to protect its partners. Apple, Google and Oracle did not immediately respond to requests for comment.

US attorney-general Merrick Garland said the decision “enables the justice department to prevent the Chinese government from weaponising TikTok to undermine America’s national security”. Deputy US attorney-general Lisa Monaco added that the “next phase of this effort — implementing and ensuring compliance with the law after it goes into effect on January 19 — will be a process that plays out over time”.

TikTok has said that any spin-off would be technologically unfeasible, while Beijing has previously indicated that it would oppose any sale.

The ruling was handed down by the court shortly after Trump said on Friday that he had discussed TikTok on a call with China’s President Xi Jinping. It was the first call between the leaders in four years.

The Supreme Court’s ruling upholds one of the boldest legislative moves of Biden’s term just days before the Democratic president leaves the White House.

On Thursday, Trump’s incoming national security adviser Mike Waltz said that the legislation “allows for an extension as long as a viable deal is on the table. Essentially that buys president Trump time to keep TikTok going.”

Chinese officials have held preliminary discussions about whether billionaire Elon Musk — now a close ally of Trump — could broker a deal for the platform’s sale, the Financial Times reported this week.

Some potential buyers and partners have been circling and lobbying Trump. These include Frank McCourt, an American media and sports businessman, who has established a consortium of investors that would bid for TikTok through his non-profit entity, Project Liberty.

TikTok chief Chew has mounted a charm offensive to cement Trump’s backing, including plans to attend a “victory rally” for the president-elect in Washington on Sunday and his inauguration on Monday, according to two people familiar with the matter. 

The TikTok legislation, passed with strong bipartisan support last year, was spurred by concerns that the popular video platform could be wielded by Beijing for espionage or to spread propaganda.

Even though China “has not yet leveraged its relationship with ByteDance Ltd to access US TikTok users’ data”, the top court said, there was “no basis for concluding that the government’s determination that China might do so is not at least a ‘reasonable inferenc[e] based on substantial evidence’.” 

TikTok asked the Supreme Court to hear its case after a US appeals court rejected its challenge to the law, as well as its subsequent request to halt the measure pending further court proceedings.

The social media app sought to throw out the law by arguing it was unconstitutional and that it violated First Amendment protections for free speech.

Additional reporting by Aime Williams in Washington and Stephen Morris and Michael Acton in San Francisco