You can tell Patrik Hermansson has been undercover for a year. The 26-year-old Swedish operative seems unwilling to give much away. Even his outfit – black shirt, black jeans, black trainers – betrays nothing. We meet at a cinema in Shoreditch, east London, where Hermansson will at last be the centre of attention: Undercover in the Alt-Right, his documentary about infiltrating white nationalist circles and exposing key figures, is being screened here for the first time.

Hermansson, now working as a researcher for the anti-extremism group Hope Not Hate, unmasked a number of leading far-right agitators from the UK and the US, mapping their networks. Most of the film was made with a microphone and camera hidden beneath his shirt. One prominent alt-right American publisher is caught on record bragging about links to the White House. Anonymous members of the military are spotted at a secret meeting in Seattle, full of heavily armed Holocaust deniers. The film’s revelations have created splits and distrust among international alt-right groups.

Pushing a sugar bowl around the table, Hermansson is more comfortable discussing the trials of his assignment than the triumphs. He recalls a slip-up when he introduced himself to his new associates with his real name. “It’s such an instinctive thing,” he laughs. (He “kind of saved it” by explaining at length how Swedes use their middle names.)

He first moved to London in 2015 as a student. Studying political science at UCL, he came to the attention of Hope Not Hate, which sent him on a mission to infiltrate a global network of Holocaust denial, extreme racism, misogyny and white nationalism.

Zeal for such ideas is growing among young men online. They have been emboldened by Donald Trump’s brand of right-wing populism. Racist and sexist views have new branding: “alt-right” is a modern term for movements that existed long before the arrival of grubby internet forums and social media.

The climax of Hermansson’s documentary is the murder of Heather Heyer, during the white supremacist “Unite the Right” rally in Charlottesville, Virginia, in August 2017. Hermansson was at the rally when a self-described neo-Nazi, James Fields, accelerated his car into a crowd of counter-protestors. Notoriously, Trump referred to “very fine people on both sides” – a watershed moment for the alt-right activists, who were encouraged by what they interpreted as sympathy for their cause.

Before arriving in London, Hermansson had been volunteering with Hope Not Hate’s sister organisation, Expo, in his home city of Stockholm. He had grown up watching the nationalist Sweden Democrats gain ground. They are now the country’s third largest party.

He encountered a far-right group during a Pride march; they tried to block his way and waved homophobic banners. “These things add up,” says Hermansson, who is gay. “You see something ugly and you want to do as much as possible to stop it.”

This is how, from October 2016 to September 2017, he came to live as “Erik Hellberg”. His alter ego was very similar to himself: a young, Swedish politics student. “You want to lie as little as possible – because every time you lie you have to remember that you lied.”

With his blond hair and blue eyes , “Hellberg” was embraced by UK far-right figures – Scandinavian heritage is fetishised in such circles. One bizarre clip shows them drinking mead from a ceremonial Viking horn, raising it to the sky in a prayer to the Norse god Odin.

Hermansson wheedled his way into this world by teaching Swedish in Soho to one of Britain’s most notorious alt-right figures. Stead Steadman, a middle-aged man behind the UK far-right group London Forum, invited Hermansson to tutor him at his favourite café, the Nordic Bakery, once a week.

Much of his interloping was in Soho, where he would also socialise with university friends. He feared his two worlds would collide. “It’s strange living two separate lives,” he says. It is also “lonely at times”. Only his boyfriend and his Hope Not Hate supervisor knew his secret. “You have very little contact with your real self,” he adds. “Everything’s an act.”

There were comical moments too. Fearing a mole, the London Forum tasked Hermansson to run background checks on new members, having somehow neglected to vet him first. “It’s super ironic and it was hilarious when I got asked,” he grins.

Later, he was invited to speak at a secret alt-right conference in the US. Not wishing to promote hate speech, he instead chose the topic of infiltration into the alt-right. “I stuck to that, mostly because it’s really funny,” he says, with another smile.

Yet he also grew numb to the daily hate. Suggesting that the Holocaust actually happened was a joke over a pint for these people. He witnessed a standing ovation for the 2017 massacre at Pulse, a gay club in Orlando. The “Jewish Question” is so often discussed it’s abbreviated to “JQ”.

“In the beginning, you are shocked about everything. And after a while you are shocked about nothing… how easy it is to normalise is one of the scariest bits.”

Hermansson became increasingly stressed about “screwing it all up” and with enough material on record, went public in a piece in the New York Times in September 2017. Security was arranged – his London address is secret. He chooses to ignore “the crowd of anonymous accounts” that now condemn him on Twitter.

Why go through all of this, as an unpaid volunteer? Why give up his safety?

“Lots of their ideas are becoming mainstream,” warns Hermansson. “Although they are very old ideas, they have been repackaged by the alt-right and taken up again by mainstream politicians.”

Undercover in the Alt-Right is now available on Amazon Prime Video.