Cloudflare, one of the internet’s most powerful companies, is storing the online content of a banned neo-Nazi terrorist organisation on UK soil and illegal Holocaust denial content across Europe.

The UK government is now facing pressure to act, previously being unable to because websites carrying extremist content have relied on US-based servers that are beyond its jurisdiction.

Researchers for the anti-extremist group Hope Not Hate found that San Francisco-based Cloudflare, which protects websites from cyber attacks, operates using “edge servers” that store web content locally on servers based in the UK and elsewhere in Europe.

The web content of NS131, a neo-Nazi terror organisation banned in the UK and associated with National Action, the first far-right group to be proscribed as a terrorist organisation by the British government, is being stored by Cloudflare in London and Manchester and delivered to users in the UK.

Five months ago a banning order against National Action, an antisemitic white supremacist group that celebrated the murder of Labour MP Jo Cox, was broadened to include two aliases used by the group, one being NS131.

The order means it is a criminal offence to belong to or invite support for NS131, known for its fetishisation of violence, admiration of Hitler and extreme antisemitism. Yet NS131’s website remains online and protected from attack by Cloudflare.

Joe Mulhall, a senior researcher of Hope not Hate, said: “Cloudflare is protecting neo-Nazi websites using technology housed in Britain. It’s been likened to guarding the podium at a Nazi rally.

“It’s spreading virulent racial hatred direct into the UK, at a time when we’ve suffered four terror attacks over 12 months, including one from a far-right extremist.”

It follows the recent conviction of Darren Osborne, jailed for life for his murderous attack on Muslims in north London last June. On Monday a white supremacist who tried to join National Action, Ethan Stables, from Cumbria, was convicted of preparing act of terror and making threats to kill.

Hope not Hate found other groups currently enjoying protection from Cloudflare’s systems include major neo-Nazi forum Stormfront, key “alt-right” websites such as Richard Spencer’s alt-right.com, along with European far-right parties like the British National party and the Swedish Democrats.

Among the Holocaust denial websites using Cloudflare is the notorious Committee for Open Debate on the Holocaust and its integrated publishing arm Castle Hill Publishing, established by convicted denier Germar Rudolf.

John Mann MP, chair of the all-party parliamentary group against antisemitism, said: “If Cloudflare wishes to continue to be the industry-standard for website protection, it is clearly going to have to raise its own standards. Not only is there a business case for doing so, there is a legal and moral imperative.”

Danny Stone, director of the Antisemitism Policy Trust, said: “If banks, publishers, couriers or any other type of international company were hosting illegal or racist documentation in the UK or Europe, we would not stand for it. Just because a company operates in the online space, it should not be afforded special treatment.”

For decades, European extremist organisations have circumvented domestic legislation relating to hate speech and Holocaust denial by hosting their sites “offshore” – such as in the US – where free speech provisions prevent law enforcement from tackling their extremist content.

Yet Cloudflare’s use of edge servers that cache clients’ content inside Europe has, according to Mulhall, raised the prospect that they are breaking laws by storing content that breaches domestic legislation.

Doug Kramer, of Cloudflare, said: “Cloudflare is not a host of any online content and as such does not control whether or not content stays online. Without Cloudflare, a website or internet property would still exist, it would just be more vulnerable to attack.

“We are aware of concerns about content that passes through Cloudflare’s network and we work hard to comply with the law and cooperate with courts and law enforcement. Throughout our history as a company, we have been transparent and upfront about how we think about these concerns. We aim to address content issues in a thoughtful way and do not believe it is our place to dictate the types of content that are online.”