Haris Theoharis resigned as parliamentary spokesman for centrist To Potami on Monday.
Theoharis had raised objections to the way the party is being run at Potami’s congress over the weekend.
He questioned the decision of its leader, Stavros Theodorakis, to pursue closer ties with PASOK. Theodorakis was re-elected as Potami leader on Sunday.
“I hope that with your renewed mandate you will take the initiatives necessary to change the party’s fate before it is too late,” said Theoharis in his resignation letter.
Theoharis, the former general secretary for public revenues, said he will remain as an MP of the small pro-reformist party.
His position as parliamentary spokesman will be filled by fellow MPs Giorgos Amyras and Spyros Danelis.