Twitter faces a landmark legal challenge after the social media giant failed to remove a series of hate-filled tweets reported by users in what could be a turning point in establishing new standards of scrutiny regarding online antisemitism.

The California-based company, owned since last year by Elon Musk, was alerted to six antisemitic or otherwise racist tweets in January this year by researchers at HateAid, a German organisation that campaigns for human rights in the digital space, and the European Union of Jewish Students EUJS but did not remove them from its platform despite the tweets apparently clearly contravening its own moderation policy.

Four of the tweets denied the Holocaust in explicit terms, one said “blacks should be gassed and sent with space x to Mars”, while a sixth compared Covid vaccination programmes to mass extermination in Nazi death camps. All were reported in January but Twitter ruled that three of the tweets did not violate its guidelines and failed to respond to the other reports, the legal action claims.

HateAid and the EUJS applied earlier this year to a Berlin court to have the tweets deleted, arguing the tweets broke German law and that Twitter had failed to meet contractual obligations to provide a secure and safe environment for its users.

Twitter has received notice of the legal action and has since acted to block some of the offending tweets.

Avital Grinberg of the EUJS, who reported some of the tweets, said the decision to take legal action had been taken out of “despair, disappointment and anger”.

“All our efforts and advocacy have led nowhere and Twitter has become a space where antisemitism and Holocaust denial is just growing and growing. This is so much bigger than us, so we needed the biggest and strongest tool that democracy has to offer and that is the law,” Grinberg said.

Experts in extremist violence said the evidence that online hate encouraged physical attacks on targeted minorities was undeniable.

Repeated studies have detected a huge surge in antisemitic online content since the Covid-19 pandemic while in 2022 the Anti-Defamation League in the US tracked the highest number of antisemitic incidents in the US since they started recording in 1979.

In the UK, the Community Security Trust recorded the fifth highest total since 1984. The interior ministry of Germany has also logged record highs in antisemitic crimes over recent years.

Jakob Guhl, of the Institute for Strategic Dialogue in London, a thinktank specialising in countering extremism, said: “Hateful and conspiratorial online ecosystems are a very significant factor in the radicalisation of individuals towards antisemitic worldviews. They are also key to the mass proliferation of narratives which seek to hold Jews responsible for the world’s ills. But this is not only an issue of online abuse and harassment. It can often translate into offline violence as well.”

Twitter has faced repeated accusations of failing to act against online hate in recent years but these have intensified since Musk took over in October. The billionaire, who has described himself as a “free speech absolutist”, restored the accounts of thousands of users who had been banned from the platform, including white supremacists with a history of involvement in neo-Nazi propaganda.

At the same time, Musk dissolved Twitter’s independent Trust and Safety Council responsible for advising on tackling harmful activity on the platform and dramatically cut staff, reportedly including those working on content moderation. Others have resigned.

Research published by the Institute for Strategic Dialogue (ISD) earlier this year found a major and sustained spike in antisemitic posts on Twitter since the company’s takeover by Musk, with the volume of English-language antisemitic tweets more than doubling.

Analysts detected 325,739 English-language antisemitic tweets in the nine months from June 2022 to February 2023, with the weekly average number of antisemitic tweets increasing by 106% when comparing the period before and after Musk’s acquisition. The rate of creation of antisemitic accounts more than tripled in the period after Musk’s takeover.

Musk himself caused controversy in May by describing Jewish philanthropist George Soros, a frequent target of abuse online, as wanting “to erode the very fabric of civilisation”, a frequent and historical antisemitic trope.

Repeated efforts by the Guardian to contact Twitter were unsuccessful. An email containing a detailed list of the alleged failures sent to Twitter’s press relations office was answered by an automated response of a poo emoji.

Josephine Ballon, the head of legal at HateAid, said the aim of the legal action was to force Twitter to take more responsibility for content on the site.

“Freedom of expression does not just mean the absence of censorship but ensuring that Twitter is a safe space for users who can be free of fear of being attacked or receiving death threats or holocaust denial. If you are a Jewish person on Twitter then the sad reality is that it is neither secure nor safe for you,” Ballon said.

“We are not demanding anything unreasonable … Just that their moderation is good enough to take down this very dangerous content. This would signal to their users that Twitter care about keeping them safe.”

The tweets reported by HateAid and the EUJS to Twitter in January included two from accounts that went on to post dozens of explicitly antisemitic tweets over subsequent months, highlighting the consequences of Twitter’s failure to act promptly.

One account – @Royston1983 – posted at least 20 further tweets containing antisemitic content since January including further explicit denial of the Holocaust alongside familiar conspiracy theories alleging predatory Jewish or Zionist plots and supposed control of the media, schools and banks.

The account has also posted falsified quotes from leading prominent Jewish figures, swastikas and tributes to historic antisemitic conspiracists. The account, which has more than 2,000 followers, frequently posts propaganda for extreme right-wing British groups and hostile invective about migrants to the UK.

The tweet reported by HateAid on 11 January had been posted by @Royston1983 in November, and read: “The holocaust is a lie, everything they told you about the holocaust is a lie, the jews have taken credit of footage that has never happened to them.” Twitter found this tweet did not violate their guidelines.

Twitter has said it is “committed to combating abuse motivated by hatred, prejudice or intolerance, particularly abuse that seeks to silence the voices of those who have been historically marginalised” and that “behaviour that targets individuals or groups with abuse based on their perceived membership in a protected category” is prohibited.

The company’s code specifically mentions the Holocaust:

“We prohibit targeting individuals or groups with content that references forms of violence or violent events where a protected category was the primary target or victims, where the intent is to harass. This includes, but is not limited to media or text that refers to or depicts genocides, (eg the Holocaust).”

HateAid on 11 January also reported another tweet by user @Abdulla74515475, which was posted on 18 December and claimed that the holocaust was “the biggest lie of the 20th century” and that “no one killed 6 million Jewish people”. Twitter found this tweet too did not violate guidelines.

The company reached the same conclusion with another tweet posted by account @Cologne1312 in January, and reported two days later, which included an image of the gates of Auschwitz death camp with the infamous motto Arbeit macht frei (work sets you free) replaced with Vaccines Will Make You Free.

HateAid say there was no response from Twitter to the other three tweets reported in January. These included calling, in German, for “Blacks to be gassed and sent to mars with space x” posted on 17 December, and another posted three days later that said “These evil Jews are lying about the Holocaust. The Holocaust was done to Jews by Jews. Forever the Jews are murdering each other blaming Germans”.

The third was posted by @RosemaryOConne6 on 5 January and read “The Jews not going into the police business apparently. This applies to the holocaust lie. Jews say 9 million, then over count, it’s 6 million like lying criminal changing his story.”

The tweet was part of a longer chain claiming that evidence for the Holocaust was limited. Since being reported in January, the account has continued to post antisemitic hate, calling Jews of “spies” and “serial killers” who hate “whites” and are conspiring with “non-whites” to ”destroy us”.

The account @RosemaryOConne6 also accused Jews of fomenting the war in Ukraine and being responsible for train derailments in the US.

Twitter were notified by the German court of the legal action earlier this month (June) and, should they decide to defend the action, a judge will set a date for a hearing later this year.

On 15 June, HateAid and the EUJS were informed by Twitter that the company had moved to block the offending tweets, though some have only been hidden from users in Germany where there are strict laws on Holocaust denial and demonstrations of Nazi sympathies. At least one account has now been suspended.

A series of studies has underlined the shortcomings of Twitter’s efforts to moderate content. A report published earlier this month by the UK-based Centre for Countering Digital Hate showed that Twitter failed to act on 99% of hate posted by subscribers to Twitter Blue, the paid-for service that shows a blue tick when, according to Twitter, “an account of public interest is authentic”.

The report accused the platform of allowing blue tick subscribers to break its moderation policies “with impunity” and indiscriminately boosting tweets without regard for their content.

Researchers collected tweets promoting hate from 100 Twitter Blue subscribers, which were then reported to the platform using Twitter’s own tools for flagging hateful conduct.

Four days after reporting the tweets, researchers found Twitter had failed to act on 99% of the posts and 100% of the accounts remained active. In the one instance that Twitter removed a hateful tweet, the account from which it was tweeted remained active.

Twitter also failed to act on tweets containing racist, homophobic, neo-Nazi, antisemitic or conspiracy content, the report said.

Grinberg said: “Twitter and other social media platforms are breeding grounds for antisemitism, racism and xenophobia and this has a massive impact on the safety of all marginalised minority communities. This is really not just about the Jewish community.”

Other reports from the CCDH and the Anti-Defamation League claimed the volume of racist content on Twitter had risen steeply in the months after Musk’s takeover in October.

Musk has said that allegations of increased hate speech were “utterly false” and that “hate speech impressions” had dramatically fallen on the platform since he took over.

In December, Twitter suspended the account of Kanye West after he posted an image of the Star of David with a swastika inside. The rapper’s account had already been suspended once for antisemitic posts.

The ISD research found that proportion of antisemitic content removed by Twitter appeared to have increased in the period since the takeover, with 12% of antisemitic tweets subsequently unavailable, compared with 6% before the takeover.

But this potential increase in removal rate has not kept pace with the increase in overall antisemitic content, with the result that hate speech remains more accessible on the platform than before Musk’s acquisition, the ISD said.

Despite Musk’s claims that “hate tweets will be max deboosted & demonetized” – suggesting they would not be algorithmically recommended to users on their news feeds and would not be able to be displayed as adverts or able to generate revenue – the ISD found little change in the average levels of engagement or interaction with antisemitic tweets before and after the takeover, the organisation said in a statement.

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