* Former leader complains about Prime Minister Najib

* Mahathir remains an influential figure in Malaysia

* Najib has been tightening his grip on UMNO  

Malaysia's former premier Mahathir Mohamad said on Monday he is quitting the ruling United Malays National Organisation (UMNO) party, as it is being seen as "supporting corruption" under Prime Minister Najib Razak's leadership.

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," Mahathir told reports at a press conference. "I feel embarrassed that I am associated with a party that is seen as supporting corruption - it had caused me to feel ashamed."

Najib's office had no immediate comment on Mahathir's statement.

The 90-year-old Mahathir, premier from 1981 to 2003, said he would not set up a new party or join another party.

"I am trying to make sure that Najib will not continue to be the PM of this country," he said.

When asked if others will follow suit in leaving UMNO, the leader said: "If they wish to follow, they are welcome but I am not campaigning for that."

Najib has been tightening his grip over UMNO, amid growing calls by detractors in the party, and the opposition, for him to step down over allegations of graft at 1MDB and revelations of the transfer of funds into his bank accounts.

Internal strife

The internal strife has damaged UMNO, the leader of all the country's ruling multi-ethnic coalitions since Malaysia's formation in 1957.

Najib's supporters forced Mahathir's son Mukhriz to step down as chief minister of the northern state of Kedah this month.

UMNO's supreme council, the party's top decision-making body, last week suspended its deputy president Muhyiddin Yassin for criticising Najib.

Muhyiddin is seen as a close ally of Mahathir.

Najib's greatest threat comes from within UMNO, if members decide he cannot lead the party to victory in elections due by 2018.

In July, the Wall Street Journal reported that investigators looking into indebted 1MDB had found that nearly $700mn had been transferred into a personal bank account in Najib's name.

Najib denies wrongdoing or taking any money for personal gain. Malaysia's Attorney General cleared Najib of any criminal offence or corruption last month, and called for all investigations to be closed.

But the prime minister still faces pressure after international investigators said they were looking into 1MDB.