Two prominent anti-Muslim campaigners have been denied entry into the UK ahead of a conference held by Britain’s newest and most active far-right group.

Martin Sellner, from Austria, and the Hungarian Abel Bodi were due to attend the private Generation Identity conference in London on Saturday. They were detained at Stansted airport – the second time in a month that Sellner, the leader of Generation Identity, has been prevented from entering the UK by border officials.

Sellner, 29, was also the ringleader behind a controversial “Defend Europe” campaign last summer, responsible for targeting boats attempting to rescue migrants in the Mediterranean.

Lauren Southern, a prominent Generation Identity activist who was refused entry to the UK at the border last month, confirmed the latest developments. She tweeted: “Martin Sellner has been detained as well as Hungarian identitarian Abel Bodi who were scheduled to attend a private conference in the UK.”

The event, being held at a closely guarded location, was arranged by Generation Identity United Kingdom and Ireland (GI UK), which has emerged over the past few months as the domestic branch of an expanding far-right network active in 13 countries.

Campaigners say GI UK has adopted a different approach to other anti-Muslim groups such as the English Defence League (EDL), with a primarily young and tech-savvy membership.

“GI’s more ‘respectable’ appearance, slick social media work, distinct iconography and disciplined public face masks an extreme ideology. The core beliefs of GI, such as ‘ethnopluralism’ and ‘remigration’, are far more extreme and race-based than anything posited by groups like the EDL,” said Joe Mulhall of Hope Not Hate.

The anti-racism campaign group says GI UK has conducted at least 88 UK actions and meetings since July 2017 in cities including London, Manchester, Portsmouth, Birmingham, Belfast, Wrexham and Glasgow.

But it remains relatively small: GI UK has just 7,445 followers on Twitter and its first major demonstration, held in Hyde Park in London last month, attracted 40 people.

A report released on Saturday by Hope Not Hate claimed GI UK’s potential belied its size. “GI UK and Ireland does pose a threat that needs to be understood, not least to make sure it fails to grow as its counterparts on the continent have done,” it said. “It has the potential to set new expected norms for young far-right activists and standards for branding and professionalism.”

The report’s release came before another far-right gathering in London featuring an anti-immigration European group accused of racialising sexual violence. The group, named #120db and based in Germany, is named after the sound volume of a pocket alarm and claims to highlight “imported violence”, which it defines as sexual violence committed by migrants.

Members including those linked to Generation Identity are to give a talk in Hyde Park on Sunday to “represent victims of mass migrant abuse”, a move that has drawn heavy criticism.