Maverick lawmaker Wong Yuk-man announced he was quitting People Power, the radical party he founded in 2011, with immediate effect.
Wong sent three e-mails on Monday morning to fellow lawmaker and party member Albert Chan Wai-yip informing him of his decision to resign from the party, its standing committee and the Anti-CY Alliance formed by its members.
Wong did not explain his decision in the e-mails.
Chan, a close ally of Wong for many years, said he was "totally astonished" as Wong had not informed him of the decision in advance.
"In recent meetings, he did not raise any issue that showed his dissatisfaction," Chan said. "But he did criticise fellow party members severely during his online radio show."
The conflict between Wong and the party was understood to be related to a earlier split between him and Stephen Shiu Yeuk-yuen.
Shui, an online broadcaster and People Party financial supporter, has publicly stated his disappointment with Wong’s vague stance towards the possibility of aggressive behaviour during the planned Occupy Central protest.