English Defence League leader Tommy Robinson and party co-founder Kevin Carroll have quit the far-right group.
In a shock announcement, counter-extremism think tank Quilliam said the pair had left EDL "because they feel they can no longer keep extremist elements at bay".
Quilliam said it had "facilitated" Mr Robinson's departure from the group, which he and Mr Carroll had set up in 2009 to protest against what they saw as the Islamisation of Britain.
Counter-racism group Hope not Hate said it welcomed the move with “cautious optimism”.
Director Nick Lowles said: “We celebrate Quilliam’s efforts here, but only a complete renunciation of the violence and hatred the EDL leaders have promoted, and a turning away from the anti-Muslim rhetoric they have championed, will be enough for the many thousands who have suffered from the EDL’s ugly actions over the past three years.”
Tell Mama, a group which monitors anti-Muslim attacks, also welcomed Mr Robinson's "defection".
Director Fiyaz Mughal said: "The real question is whether Tommy Robinson has changed from just a few days ago when he was at the heart of the EDL machinery and where his messages undermined the work of anti-racist campaigners."
He urged Mr Robinson "to apologise to our nation for the millions of pounds spent on policing costs and for the tensions in communities he has caused".
Mr Robinson, whose real name is Stephen Yaxley-Lennon, and Mr Carroll are due to stand trial later this month for allegedly attempting to defy a ban on marching to the scene where soldier Lee Rigby was killed, in June.