A clandestine meeting of Europe’s “hipster far right” was plunged into chaos last week when a prominent member of the movement’s British wing resigned after being shown evidence that a colleague had neo-Nazi connections and had been linked to racist attacks.

Up to 200 teenagers and twentysomething members of Generation Identity, including a large group from the UK, had gathered at a secret location in rural south-central France to formulate the next stage of its campaign to halt what the group calls the “great replacement of white people with black and ethnic minorities”.

But the meeting, in the village of Saint-Didier-en-Velay, was disrupted after the Observer forwarded information to Generation Identity members revealing that a prominent figure in the pan-European far-right youth movement had neo-Nazi connections and had been linked to racist stabbings.

Within an hour of the information being sent, Generation Identity’s British co-leader, Tom Dupré, the son of an insurance broker from Sevenoaks, Kent, and described by anti-fascists as one of the most capable figures on the UK far right, had resigned and left the camp, saying he was “disgusted” that the group had links to neo-Nazis.

Dupré, 23, a former City banker who studied psychology at Bristol University, told the Observer: “I’ve just left; obviously, this information will cause mayhem at the camp. I’ve spoken to a couple of people and they are extremely concerned. If you’re talking about Islam and immigration, you need a complete brick wall to keep out your racists, your Nazis. If anything gets through, the whole project is a waste of time. I’m appalled. I’ve resigned. My personal view is that others will leave [the group].”

Simon Murdoch of the anti-fascist group Hope not Hate said Dupré’s resignation could prompt the UK branch of Generation Identity – the second largest contingent at the gathering last week – to collapse.

He said: “Dupré is one of the most competent far-right leaders to emerge in recent years and is the sort of person the UK far right has really struggled to attract. His leaving GI UK will be a major blow for them as they lack any depth of quality activists. Whether the UK branch will survive without him remains to be seen.”

The branch is part of a 13-nation European Generation Identity movement with significant numbers in France, Germany and Austria. It is infamous for its Defend Europe mission, which sought to block refugee rescue ships in the Mediterranean last summer. In March, one of its most senior figures, Martin Sellner from Austria, was detained and deported when he attempted to enter Britain and address the group’s first UK meeting.

Dupré, speaking before he had received the information that a senior GI colleague had neo-Nazi links, said: “Our core ideas are [about] the great replacement, which means [being against] sustained mass immigration over a long period.” He also said that the group was focused on what it called the “Islamification” of Europe. “Certain Islamist sects shouldn’t be here and are obviously illegal immigrants,” he said.

Despite its extreme views on immigration and Islam, the group says that it denounces racism and nazism and does not condone violence. During the “summer camp”, members wore T-shirts bearing a French translation of the ancient Greek poet Homer – “Du combat seuls les lâches s’écartent” or “they are cowards that depart from battle” – and took part in seminars interspersed with “physical training” sessions, during which plastic knives were used and street riots simulated.

The group’s ideology includes the “reconquista” of Europe – the Spanish term for reconquest – which describes how non-Christians were expelled or converted when Christian forces took over Muslim-ruled parts of Spain during the 15th century.

Critics of Generation Identity say that behind the respectable appearance of its members and slick social media work, its central beliefs are more extreme and race-based than anything proposed by groups such as the English Defence League.

In June, Facebook permanently banned Generation Identity for violating its policies.