Former Malaysian prime minister Mahathir Mohamad resigned from the party he led for more than 20 years following sharp differences with prime minister Najib Razak.
The former prime minister has been a harsh critic of the leaders of the ruling United Malays National Organisation (UMNO) especially prime minister Najib.
"I won't call it UMNO anymore, this is Najib's party. - it had caused me to feel ashamed," Mahathir said at a news conference.
Mahathir said his son, former Kedah chief minister Mukhriz Mahathir, will also quit the party.
Mahathir and Mukhriz have been in the frontline of political attack of Najib, whose reign is tainted with allegations of corruption and the poor state of economy.
The country's longest-serving prime minister has been under investigation for making seditious remarks about the government.
The criminal defamation investigation was slapped on Mahathir after he lambasted Attorney General Mohd Apandi Ali in a recent blog post for clearing prime minister Najib of wrong doing in a corruption probe.
The country's establishment and top leaders of the ruling party lashed out at the post saying many of the comments the 90-year-old leader made were seditious in nature.
Mahathir, who quit as prime minister in 2003 after a more than 20-year reign at the top, had resigned from the party in 2008 fallowing disputes with prime minister Abdullah Ahmad Badawi.
He returned to the party when Najib became prime minister in 2009.
Mahathir said the Unmo had become 'incorrigible', citing the suspension of former deputy prime minister Muhyiddin from the party for criticizing Najib.