Demosisto on Thursday said that prominent member Derek Lam has quit the party after violating its money-handling procedures.
The party, founded by former student leaders Joshua Wong, Nathan Law and others, said it had dismissed Lam as a standing committee member even before he submitted his departure request.
The party said it had suffered no financial loss.
Lam, a CUHK theology student, had worked as a part-time assistant to Law when the Demosisto leader became a lawmaker.
On his personal Facebook page, Lam said he was full of "apologies and thanks" to the party, and said it's the start of a new journey.