KUALA LUMPUR - Former Malaysian premier Mahathir Mohamad announced Monday he was quitting the ruling party, saying it had degenerated into an organisation whose sole purpose was to protect scandal-plagued Prime Minister Najib Razak.

Mahathir has for months been a leading voice calling for the ouster of Najib, who is under pressure over allegations that billions of dollars had been pilfered from state firms, and for accepting a mysterious $681 million sum from an overseas source.

"I want to leave UMNO because it is no longer UMNO," Mahathir was quoted saying, referring to the United Malays National Organisation, which he led from 1981-2003. "It is a party dedicated to protecting Najib. I can't be a member of such a party," the news portal Malaysiakini quoted him saying. Authorities in several countries are currently investigating global money movements centering on a company launched by Najib called 1Malaysia Development Berhad (1MDB).

Swiss authorities have said they believe $4 billion may have been stolen from Malaysian state firms and had frozen millions in accounts linked to 1MDB.

Najib responded to the growing uproar last year by sacking Malaysia's attorney general and installing his own appointee, who has brought domestic investigations into the scandals to a halt.

He also purged his cabinet of critics over the affair and his government has arrested whistle-blowers and shut down media outlets that reported on the scandal.

Last week, authorities began blocking access to The Malaysian Insider, a leading news portal, after it ran a story on the 1MDB affair.

Najib's attorney general recently provoked howls of protest by clearing him of any wrongdoing in the $681 million donation.

Through it all, the irascible Mahathir has continued to savage Najib, accusing him of corruption, criminal activity and of bribing politicians to maintain their support and to avoid charges.

But his campaign has failed to gain traction in the long-ruling UMNO, which runs on a system patronage and often outright corruption, say political analysts.