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The social media company had hoped a federal appeals court would agree with its argument that the law was unconstitutional because it represented a "staggering" impact on the free speech of its 170 million US users.

But the court upheld the law, which it said "was the culmination of extensive, bipartisan action by the Congress and by successive presidents".

TikTok says it will now take its fight to the US Supreme Court, the country's highest legal authority.

The US wants TikTok sold or banned because of what it says are its owners links to the Chinese state - links TikTok and parent company Bytedance have always denied.

The court agreed the law was "carefully crafted to deal only with control by a foreign adversary, and it was part of a broader effort to counter a well-substantiated national security threat posed by the PRC (People's Republic of China)."

But TikTok said it was not the end of its legal fight.

"The Supreme Court has an established historical record of protecting Americans' right to free speech, and we expect they will do just that on this important constitutional issue," a TikTok spokesperson said in a statement.

They added that the law was based on "inaccurate, flawed and hypothetical information" and a ban would censor US citizens.

Donald Trump's victory in the 2024 US Presidential Election may also present a lifeline for the app.

Despite unsuccessfully attempting to ban TikTok during his first term in 2020, he said in the run-up to the November elections he would not allow the ban on TikTok to take effect. When and why could the US ban TikTok? Is TikTok really a danger to the West?

Trump will be inaugurated on 20 January - the day after the law says TikTok must be be banned or sold.

However, it remains to be seen whether he will follow through on his pre-election vow.

Professor James Grimmelman of Cornell University said the president-elect would be "swimming upstream to give TikTok a reprieve".

"The anti-China sentiment in the US Congress is very strong, so there are now substantial constituencies in both parties that want TikTok to be restricted from the US market," he told BBC News. Users and rivals

The court case has been closely watched both by those who use TikTok- and the app's rivals.

Tiffany Cianci, a small business advocate and TikTok creator, said she was "not shocked" by Friday's decision - but told BBC News she would not be shifting her TikTok content or presence to the platform's rivals, such as Instagram.

"I'm not going to do what they want and take my content to their platforms where it's not as successful where it's more likely to be censored, where I am more likely to have less control over my audience," she said.

Nonetheless, other platforms are positioning themselves for a post-TikTok social media landscape.

Meta, which owns Facebook as well as Instagram, WhatsApp and Threads, has sought to build rivals to TikTok's short form videos within its own apps, and made changes that users have likened to TikTok amid questions over the app's US future.

Jasmine Enberg, principal analyst at eMarketer, said there would be "major upheaval" if a TikTok appeal were to fail at the Supreme Court and a ban was enforced.

She said this would be "benefitting Meta, YouTube and Snap, while hurting content creators and small businesses that rely on the app to make a living."

But TikTok won't be easily recreated, said Cory Johnson, Chief Market Strategist at Epistrophy Capital Research. Johnson said deep learning models power TikTok's recommendation engine.

"Enabling such complex AI and big data processing at TikTok's immense scale requires a colossal and expensive technical infrastructure," Johnson said.

He said TikTok's hyper-targeting and China's data laws pose significant risks, and pointed to Elon Musk's alterations to algorithms at his social media platform X, formerly known as Twitter, as a cautionary tale.

In the run-up to the U.S. election, Musk's political posts received more views than all U.S. political campaign ads on X's disclosure dataset, Johnson said.

"We have very real and very recent experience in America with a social media network tweaking its algorithms to favor certain voices," he added.