In banning Donald Trump, Twitter and Facebook hoped to quell complaints — from Democrats and others — that they have allowed the US president to use their platforms to sow misinformation and hate.

In doing so, however, they have amplified a complaint long made by conservatives, which is that too much political influence is now wielded by a handful of private technology companies which can decide who can and cannot reach their audiences.

The events of the past week have given ammunition to critics on both sides who want stricter regulation for social media platforms.

“The actions of the technology companies last week were legal, there is no question about that,” said Jameel Jaffer, the executive director of the Knight First Amendment Institute at Columbia University. “But they also underscore the immense power that some of these companies now have as gatekeepers to the public square.”

Facebook and Twitter both barred Mr Trump after deciding his actions during last week’s riot by his supporters in the US Capitol amounted to an incitement to violence.

But if the platforms hoped to stave off the threat of regulation from Democrats — who last week secured control of the Senate to add to the House of Representatives and the White House — they look to have been unsuccessful.

Richard Blumenthal, one of the Democratic senators who has led attempts to introduce stricter regulation on large technology companies, said the riot at the Capitol would “renew and focus the need for Congress to reform big tech’s privileges and obligations”.

The part of the law that Mr Blumenthal and his Senate colleagues want to revisit is the legal protection granted to online companies at the birth of online communication. Under Section 230 of the 1996 Communications Decency Act, internet platforms can publish and moderate content from third parties without being held legally liable for what they say.

The companies call this part of the law the “twenty-six words that created the internet”.

Critics say companies have grown to possess such power over political speech that those legal protections should no longer apply.

Both Mr Trump and Joe Biden, the Democratic president-elect have in the past called for the repeal of Section 230, and several members of Congress are working on bills that would limit when it applies. The problem is that there is little agreement on what those limits should be.

Daphne Keller, director of the Program on Platform Regulation at Stanford University, said: “Democrats and Republicans both agree they want to change Section 230, but they both want to do it for different reasons. Democrats want platforms to take down more, but Republicans want them to take down less.

“The question is whether either side has the votes to be able to get something passed.”

Lindsey Graham, the Republican from South Carolina who is the outgoing chairman of the Senate judiciary committee, has proposed only allowing companies to enjoy immunity from libel law if they sign up to a set of best-practice guidelines which would be set by an independent commission.

He is promoting a measure which would phase out Section 230 if a replacement is not agreed by 2023.

Brian Schatz, the Democratic senator from Hawaii, and his Republican colleague John Thune, from South Dakota, have jointly proposed a law which would force companies to be more transparent about how they and why they moderate content.

Both Facebook and Twitter say they would support some restrictions on Section 230, though have not said exactly what. Industry lobbyists argue that anything but the most minor of changes will make companies less likely to moderate content in an effort not to be classified as publishers.

Meanwhile, a similar debate is playing out outside the US. In the UK, Matt Hancock, the health secretary, argued that the decision to ban Mr Trump “raises questions about [social media companies’] editorial judgments and the way that they’re regulated”. In Germany, chancellor Angela Merkel called Twitter’s move a “problematic” breach of the “fundamental right to free speech”.

American lawyers say Facebook and Twitter acted legally in barring Mr Trump, since the duty to guarantee free speech under the first amendment of the US constitution applies only to the government, not private companies.

This is the case even when the companies in question have significant power over online speech.

Last year a US appeals court knocked down a complaint by Prager University, a rightwing YouTube channel, that the video-sharing site had violated its right to free speech by limiting its reach.

The judge ruled: “Despite YouTube’s ubiquity and its role as a public-facing platform, it remains a private forum, not a public forum subject to judicial scrutiny under the first amendment.”

US supporters of stricter regulation are paying close attention to what happens in the EU, where the proposed Digital Services Act would give users the right to challenge social media sites which remove their content.

Gregory Magarian, a law professor at Washington University, said: “Europe has shown that it is possible to apply incremental regulations to technology companies without strangling them or killing their business models. If it can be done in Europe and elsewhere, it can be done here.”