Donald Trump will remain blocked from Facebook after a ruling from the company’s independent oversight board that has sparked a fierce backlash from Republicans.

The decision will continue to deprive the former president of a vital mouthpiece. The panel decided Facebook should review within six months whether the freeze should stay in place permanently and criticised the company for an ad hoc and opaque decision-making process.

Trump responded by lashing out at big tech. “Free Speech has been taken away from the President of the United States because the Radical Left Lunatics are afraid of the truth,” he said, adding: “These corrupt social media companies must pay a political price, and must never again be allowed to destroy and decimate our Electoral Process.”

Facebook suspended Trump’s account indefinitely four months ago over fears he could stir further violence and unrest following the January 6 storming of the Capitol by a mob of the president’s supporters.

Later in January, Facebook asked its oversight board — a Supreme Court-style body first appointed in 2020 — to review the ban, as well as provide guidance on how the platform should treat rule-breaking content from world leaders.

The board said it upheld the decision to restrict the former president’s account because “in maintaining an unfounded narrative of electoral fraud and persistent calls to action”, he had created “a clear, immediate risk of harm”.

However, the board said Facebook chief executive Mark Zuckerberg’s ban of Trump “indefinitely” reflected flaws in the decision-making process around content moderation.

“In applying a vague, standardless penalty and then referring this case to the board to resolve, Facebook seeks to avoid its responsibilities” it said.

It is unclear whether Trump, once an obsessive user of social media with more than 35m followers on Facebook and 24m on Instagram, will be able to regain access. He has not ruled out running again for the White House in 2024.

In a blog post, Facebook’s vice-president of global affairs and communications Nick Clegg said the company believed the decision to suspend Trump was “necessary and right”.

“We’re pleased the board has recognised that the unprecedented circumstances justified the exceptional measure we took,” Clegg said.

Some Republican figures seized on the board’s decision. “Facebook is more interested in acting like a Democrat Super PAC than a platform for free speech and open debate. If they can ban President Trump, all conservative voices could be next,” wrote Republican congressman Kevin McCarthy on Twitter. Jim Jordan, a Republican representative from Ohio, said: “Break them up.”

Democrats and civil rights groups urged Facebook to permanently ban the former president and reiterated calls for tougher regulation.

Frank Pallone, chair of the House energy and commerce committee, said: “Whether [Trump is] on the platform or not, Facebook and other social media platforms with the same business model will find ways to highlight divisive content to drive advertising revenues.”

The ruling is the biggest test to date of Facebook’s 20-member oversight board, made up of academics, civil rights groups and experts across the political spectrum.

The board made several recommendations that the social network is obliged to consider but not bound to follow, including creating a team to handle the moderation of political speech from influential users.

Facebook rival Twitter banned Trump permanently in January. Google’s YouTube has suspended the former president from the video platform but chief executive Susan Wojcicki said it would lift the freeze once “the risk of violence has decreased”.

Trump has kept a relatively low profile since leaving the White House in late January and snubbed Joe Biden’s inauguration. He has made few public appearances and participated in only a handful of interviews, mostly with Fox News.

He has continued to repeat some of the unproven claims of election fraud that contributed to his bans.