Mahathir, Malaysia's longest-serving leader, remains a highly respected and influential figure and has become the fiercest critic of Najib, who is facing pressure over a graft scandal surrounding state fund 1Malaysia Development Berhad (1MDB).
"I won't call it UMNO anymore, this is Najib's party. I feel embarrassed that I am associated with a party that is seen as supporting corruption - it had caused me to feel ashamed," he told reporters at a press conference, Reuters reported.
But the leader said he would not set up a new party or join any other party.