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US Supreme Court justices appeared sceptical about TikTok’s efforts to throw out a federal “divest-or-ban” law on Friday, as the social media platform warned it could “go dark” in one of its biggest markets in nine days’ time.

The oral arguments on Friday centred on whether to allow a law to take effect that would compel TikTok’s Chinese parent ByteDance to divest the platform by January 19 — the day before Donald Trump is inaugurated as president — or face a nationwide ban.

The legislation, passed with strong bipartisan support last year, was spurred by concerns that the video platform, which has exploded in popularity among teens and now counts 170mn US users, could be wielded by Beijing for espionage purposes or to spread propaganda.

TikTok has denied the accusations and claimed the law violated First Amendment protections for free speech. Meanwhile, Trump has promised to “save the app”, and implored the top court to delay the legislative deadline to allow for “the opportunity to pursue a political resolution of the questions at issue in the case” when he returns to the White House later this month.

Either way, the court’s decision will have wide-ranging implications for free speech in the US as well as global relations with China.

During oral arguments on Friday, justices across the ideological spectrum repeatedly challenged TikTok’s arguments that the law was an assault on free speech, focusing instead on concerns the platform is used for “covert manipulation” and its data is vulnerable for harvesting by Beijing.

A lawyer for TikTok, Noel Francisco of Jones Day, said the law singled out the company “for uniquely harsh treatment, and it does so because the government fears that China could, in the future, indirectly, pressure TikTok”.

Chief Justice John Roberts, a member of the court’s conservative wing, replied: “So are we supposed to ignore the fact that the ultimate parent is, in fact, subject to doing intelligence work for the Chinese government?”

Justice Elena Kagan, part of the court’s liberal wing, conceded the company “is going to suffer some pretty severe [but] incidental effects”. If TikTok ultimately loses access to ByteDance’s algorithm as a result of a divestiture, the law still “leaves TikTok with the ability to do what every other actor in the United States can do, which is go find the best available algorithm”, she said.

Elizabeth Prelogar, the US solicitor-general, stressed the government’s national security argument. Beijing’s efforts to undermine the US by amassing “sensitive data” about Americans and its ability to compel companies to turn over such material “mean that the Chinese government could weaponise TikTok at any time to harm the United States”, she said.

She said ByteDance had already acquiesced to Beijing’s demands, alleging there was evidence it had “taken action to misappropriate data . . . to track dissidents in Hong Kong [and] Uyghurs in China”.

ByteDance also “misappropriated US data” when it admitted to having inappropriately obtained the data of two US journalists, including a Financial Times reporter, she added.

TikTok has argued a spin-off would be technically “unfeasible” before the deadline. Beijing, which would have a say, according to China’s export laws, has also said it opposes a sale and has branded the law a “blatant act of commercial robbery”. Francisco, TikTok’s lawyer, reiterated the point on Friday, saying a divestiture would be “exceedingly difficult under any timeframe”.

Asked what would happen on January 19 if the company loses this case, Francisco replied: “As I understand it, we go dark.”

The court is expected to deliver a ruling ahead of the January 19 deadline, potentially sealing the fate of a leading source of entertainment and news for young people, which has provided a livelihood to thousands of influencers, as well as attracting top advertising dollars.

Even if the court rules against TikTok, Trump could intervene once in power, though it is unclear how. The president-elect’s sudden rescue mission of the video app has come in part after he used the platform during last year’s election campaign to engage with young voters.

It also comes as Trump has said he wishes to preserve “competition” in a market dominated by Mark Zuckerberg’s Meta, after criticising the US social network as an “enemy of the people” for alleged censorship of conservative content.