Alexopoulos jumps ship before vote that could allow criminal charges to be brought against far-right Greek party's MPs

Greece's far-right Golden Dawn party has been thrown into crisis after one of its MPs broke ranks before a crucial vote by the Athens parliament that is likely to pave the way for criminal charges to be brought against the group's 18 MPs.

As the legislature prepared to cancel politicians' immunity from prosecution in the coming days – following charges that the MPs ran the neofascist force as a criminal organisation – Chrysovalantis Alexopoulos abandoned the party, citing ignorance of its activities. The extremists, whose emblem bears a resemblance to the swastika and whose leadership is prone to giving Nazi salutes, are seen as Europe's most violent political force.

"I'm sorry that I did not act earlier, and that I did not differentiate my stance," the 43-year-old wrote in a letter to parliament, explaining his decision to sit in the 300-seat house as an independent. "I reached this decision following a series of revelations regarding the activities of [Golden Dawn], which have been defined as criminal, and of which naturally I had no knowledge. My conscience no longer permits me to associate my patriotic and nationalist views with such actions and practices."

His desertion threw the once water-tight Golden Dawn into disarray, with insiders speaking of a "civil war atmosphere" in its ranks. Recent polls show support for the party – until last week austerity-whipped Greece's third biggest force – dropping precipitously.

"This is how far-right parties have disintegrated in Greece in the past," the writer Dimitris Psarras, who has followed Golden Dawn since its inception as a fringe group more than 30 years ago, told the Guardian. "Alexopoulos's decision to break ranks is the first big rupture."

Judicial sources said the MP's implicit recognition of the group's illegal activities would add credibility to the array of charges the party faces. Since the extremists were catapulted into parliament with 7% of the vote on the back of widespread fury with the political establishment almost two years ago, immigrants, leftists and gay people have been attacked, often brutally, by black-clad Golden Dawn supporters.

An in-depth investigation of the group by magistrates was triggered by the fatal stabbing of a popular leftwing musician at the hands of a party operative last September.

Six of the organisation's 18 deputies – including its enigmatic founder, the mathematician Nikos Michaloliakos – have already been accused of using Golden Dawn to operate a criminal gang and are in prison pending trial. Three others face similar charges but have been released from custody on bail.

On the basis of a 200-page report compiled by prosecutors, three more Golden Dawn MPs had their immunity from prosecution lifted last week. Among them was Michaloliakos's wife, Eleni Zaroulia, who triggered global condemnation when she described immigrants as "subhuman" in an address to parliament in 2012.

Alexopoulos' desertion sparked instant derision with the ultra-rightists denouncing the politician as a "traitor" and a "coward", and accusing him of betraying the cause. The party's spokesman, Ilias Kasidiaris, a candidate for mayor in forthcoming local elections, appeared to acknowledge that the organisation was in trouble.

"If any other party had suffered what they have done to Golden Dawn," he said, preparing supporters for the possibility of further desertions, "not only MPs would have left, but cadres and members too, taking with them their offices, doors and windows included."

Giving voice to that sentiment the mass-selling daily newspaper Ta Nea dedicated its front-page editorial to what it hoped would soon be the group's demise, describing Alexopoulos' desertion as a "positive development".