The global publishing platform Substack is generating revenue from newsletters that promote virulent Nazi ideology, white supremacy and antisemitism, a Guardian investigation has found.

The platform, which says it has about 50 million users worldwide, allows members of the public to self-publish articles and charge for premium content. Substack takes about 10% of the revenue the newsletters make. About 5 million people pay for access to newsletters on its platform.

Among them are newsletters that openly promote racist ideology. One, called NatSocToday, which has 2,800 subscribers, charges $80 – about £60 – for an annual subscription, though most of its posts are available for free.

NatSocToday is understood to be run by a far-right activist based in the US and features a swastika, a symbol appropriated by the Nazi party in the 1920s to symbolise white supremacy, as its profile picture. The full name of the Nazi party was the National Socialist German Workers’ party.

One of its recent posts suggests the Jewish race was responsible for the second world war and describes Adolf Hitler as “one of the greatest men of all time”. Within two hours of subscribing to NatSocToday for the purposes of this investigation, the Substack algorithm directed the Guardian’s account to 21 other profiles featuring similar content.

Some of these accounts regularly share and like each other’s posts. Many have thousands of followers.

Erika Drexler, a self-styled “NS [national socialist] activist” with 241 subscribers, shared posts describing Hitler as her hero and the “most overqualified leader ever”. The account is also believed to be US-based and charges $150 for an annual subscription.

Ava Wolfe, who has 3,000 subscribers and calls herself an “archivist of articles and videos about history in particular WW2” appears to be based in the UK. She has a profile which features swastikas and other Nazi imagery. An annual subscription to her Substack costs £38.

Much of the content Wolfe posts engages in Holocaust denial. About 6 million Jews died in the Holocaust, but she falsely claimed earlier this month that doctors had found that “no one was deliberately murdered by Germans” and that “death was from disease and starvation only”.

It is unclear if Drexler and Wolfe have used their real identities to post their material, or if they write under pseudonyms.

Another account, entitled Third Reich Literature Archive, with 2,100 subscribers, shared postcards purporting to be from a Nazi propaganda rally in Nuremberg in 1938, the year before the second world war began. It also charges $80 a year for a premium subscription.

The Guardian account was shown separate posts that promoted conspiracy theories about Jewish power and influence and suggested antisemitism was a myth.

The algorithm also promoted other extremist content, including newsletters relating to the “great replacement” conspiracy theory – the suggestion that there is a plot to replace white Europeans with people from other races.

There has been a sharp increase in antisemitism and Islamophobia since the beginning of the Israel-Gaza war in October 2023. Two men were killed when a synagogue in Heaton Park, Manchester, was attacked on the Jewish holiday of Yom Kippur in October last year. Fifteen people were shot dead as they celebrated Hanukah at Sydney’s Bondi Beach in December.

The chief executive of the Antisemitism Policy Trust, Danny Stone, said harmful online content often inspired real-life attacks.

As examples, Stone cited the racially motivated murder of 10 African Americans in Buffalo, New York, in 2022; a synagogue shooting in Pittsburgh, Pennsylvania, in 2018 in which 11 people were killed and the 2017 attack on a mosque in Finsbury Park, north London, in which one person was killed and several injured.

“People can be, and are, inspired by online harm to cause harm in real world,” he said. “The terrorist who attacked Heaton Park synagogue didn’t wake up one morning and decide to kill Jews; he will have been radicalised.

“Algorithmic prompts and the amplification of harmful materials is extremely serious. The Online Safety Act was supposed to address the illegal content but very little is being done about so-called legal but harmful content.”

Stone also expressed concern about online disinformation about the Holocaust.

“There has been a drop in attendance and take-up of Holocaust memorial events,” he said. “We know that knowledge already was frighteningly low.

“When you have Holocaust denial, inversion or comparisons, you are seeing, across the board, a diminishing of the memory of the Holocaust. As we are further away, with fewer survivors, the facts can get lost.

“We have to win the battle for that narrative. This online content does extreme damage because if we fail to learn the lessons of that past, we’re doomed to repeat it.”

A spokesperson for the Holocaust Educational Trust said: “Material like this that spreads conspiracy theories and Holocaust denial and which praises Hitler and the Nazis is not new but clearly its reach is increasing. The idea that Substack profits from this hateful material and allows for it to be boosted via their algorithm is a disgrace.

“We are acutely aware of the passage of time which moves us further away from the events of the Holocaust, and eyewitnesses to this history are becoming fewer in number. At the same time, antisemitism is increasing – this extremism needs to be exposed, challenged and stamped out.”

Joani Reid, the Labour chair of the all party parliamentary group against antisemitism, said she planned to write to Substack and Ofcom to ask them to address the Guardian’s findings. She said antisemitism was “spreading with impunity” and getting worse.

“We need to hold these tech companies to account because there are real-life consequences to this,” she said. “Jewish people have been complaining about this for years – saying this violence online is going to end in violence offline and that is exactly what has happened. We need to start taking this stuff far more seriously.”

Substack was contacted for comment but did not respond.

Launched in 2017, the platform has previously faced criticism for hosting newsletters that promote extremist views. Its co-founder, Hamish McKenzie addressed its decision to host Nazi content in one of his own posts on the site in 2023.

“I just want to make it clear that we don’t like Nazis either – we wish no one held those views,” he wrote. “But some people do hold those and other extreme views. Given that, we don’t think that censorship (including through demonetising publications) makes the problem go away – in fact, it makes it worse.

“We believe that supporting individual rights and civil liberties while subjecting ideas to open discourse is the best way to strip bad ideas of their power. We are committed to upholding and protecting freedom of expression, even when it hurts.”

McKenzie also said the site’s content guidelines “do have narrowly defined proscriptions, including a clause that prohibits incitements to violence”.