By Brian Fung, CNN Business

(CNN) -- Facebook can keep blocking former President Donald Trump from using its platform, the social network's court-like Oversight Board said Wednesday in a landmark move that affirmed the company's decision to issue the suspension after the January 6 US Capitol riots.

The board said it concluded that Trump's posts on January 6, which praised the rioters, "severely violated" Facebook's policies and "created an environment where a serious risk of violence was possible."

However, the board criticized Facebook for having made the suspension indefinite and said Facebook must review the decision and impose disciplinary actions such as a "time-bound period of suspension" or permanently disabling the account — sanctions consistent with Facebook's policies.

"Within six months of this decision, Facebook must reexamine the arbitrary penalty it imposed on January 7 and decide the appropriate penalty," the board wrote in its decision. "This penalty must be based on the gravity of the violation and the prospect of future harm. It must also be consistent with Facebook's rules for severe violations, which must, in turn, be clear, necessary and proportionate."

"In applying a vague, standardless penalty and then referring this case to the Board to resolve, Facebook seeks to avoid its responsibilities," the decision said.

The decision also applies to Facebook-owned Instagram where Trump has an account. Trump has almost 60 million followers across Facebook and Instagram.

The board additionally called on Facebook to "undertake a comprehensive review of Facebook's potential contribution to the narrative of electoral fraud and the exacerbated tensions that culminated in the violence in the United States on January 6."

Nick Clegg, Facebook's vice president of communications, said in a statement that Facebook was "pleased the board has recognized that the unprecedented circumstances justified the exceptional measure we took."

"We will now consider the board's decision and determine an action that is clear and proportionate," Clegg said, adding: "In the meantime, Mr. Trump's accounts remain suspended."

Trump reacted in a statement, describing the decision as a "total disgrace" and "embarrassment."

The announcement from the board was likely a disappointment to Facebook, which had seemingly hoped that it could wash its hands clean of difficult decisions and deflect them to the Oversight Board.

In a call with reporters, however, Oversight Board co-chair Helle Thorning-Schmidt pushed back against the characterization that the board had simply avoided making the tough call and deflected responsibly for the decision back to Facebook.

"I don't think we are just passing the buck here," Thorning-Schmidt said, explaining that the board did make a decision to uphold the initial suspension, but believed that an indefinite suspension was not in accordance with Facebook's own rules.

"They can't just invent new, unwritten rules when it suits them," Thorning-Schmidt added. "They have to have a transparent way of doing this."

Despite the nuance, the ultimate decision from the Oversight Board is a blow to Trump, denying him a sizable platform to use as a bully pulpit as he attempts to retain firm control of the Republican Party while out of office.

Trump was suspended "indefinitely" from Facebook and Instagram on Jan. 7, a day after his supporters stormed the Capitol in a bid to overturn the 2020 election results. Twitter and YouTube took similar steps, citing an ongoing risk of violence and incitement.

"We believe the risks of allowing the President to continue to use our service during this period are simply too great," Facebook CEO Mark Zuckerberg wrote at the time.

Later that month, Facebook asked the Oversight Board for a ruling on whether to let Trump's suspension stand, saying the significance of the matter warranted its independent review.

Amid rising scrutiny of tech platforms worldwide and growing threats of government regulation, the case involving Trump's account quickly became about much more than a single user's page.

The deliberations marked the Board's biggest challenge since its launch last fall. Civics experts said the case would be a major test of the Board's independence from Facebook and of its effectiveness at its intended job — acting as a kind of Supreme Court for content removals that would create binding precedents for Facebook to follow.

Though funded by Facebook, the Oversight Board is designed to operate autonomously from the company. In reviewing Facebook's content decisions, the Board draws its voting members from fields ranging from international and human rights law to journalism and digital rights advocacy. But critics of the group have alleged it is little more than a fig leaf meant to lend the appearance of legitimacy and accountability.

The decision to bar Trump from Facebook, if made permanent, could have vast implications, wrote Evelyn Douek, a researcher of online speech and platform moderation at Harvard Law School.

"There is no greater question in content moderation right now than whether Trump's deplatforming represents the start of a new era in how companies police their platforms, or whether it will be merely an aberration," Douek wrote in January. "What one platform does can ripple across the internet, as other platforms draft in the wake of first movers and fall like dominoes in banning the same accounts or content. For all these reasons, the board's decision on Trump's case could affect far more than one Facebook page."

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