The censorship of conservative content on YouTube is not unlawful, a US court has ruled. 

The US Court of Appeals for the Ninth Circuit ruled on Wednesday that the video-sharing platform is not compelled by the First Amendment to allow all viewpoints to be shared on the site. 

The ruling was in response to a legal challenge brought by PragerU, an American right-wing non-profit, against YouTube and Google. 

The appeals court upheld the verdict of a lower court that had dismissed the organisation's lawsuit. 

Handing down the judgement, Circuit Judge M Margaret McKeown said that free speech was not guaranteed on YouTube because it was not a public forum.

"Despite YouTube's ubiquity and its role as a publicfacing platform, it remains a private forum, not a public forum subject to judicial scrutiny under the First Amendment," she wrote. 

"PragerU runs headfirst into two insurmountable barriers—the First Amendment and Supreme Court precedent.

"Just last year, the Court held that 'merely hosting speech by others is not a traditional, exclusive public function and does not alone transform private entities into state actors subject to First Amendment constraints.'"

In a statement, PragerU CEO Marissa Streit accused the court of being afraid of Google. 

"As we feared, the Ninth Circuit got this one wrong, and the important issue of online censorship did not get a fair shake in court," Streit said.

"Sadly, it appears as if even the Ninth Circuit is afraid of Goliath — Google. We're not done fighting for free speech and we will keep pushing forward."