Former Malaysian Prime Minister Mahathir Mohamad says he is leaving the ruling party because it is “seen as supporting corruption”.
Mr Mahathir said he was “embarrassed” to be associated with the United Malays National Organisation (Umno) because of its support for current PM Najib Razak.
The former leader has been one of the fiercest critics of Mr Najib, who has been plagued by corruption allegations.
Mr Najib denies the allegations and has been officially cleared.
Mr Najib was accused of taking $681m (£479m) from a state investment fund into his personal bank account.
However, the attorney-general’s office cleared him in January, saying the money was a personal donation from the Saudi royal family.
Malaysia’s ‘mysterious millions’ – case solved?
The previous attorney-general leading the investigation into the fund was sacked last year.
At a news conference in Kuala Lumpur, Mr Mahathir, who has repeatedly called for Mr Najib to resign, said it was now “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.
“I decided that I cannot be a party to all these things, so the least I can to do is leave the party.”
But he said he would not be setting up a new party.
Analysis: Jonathan Head, BBC South East Asia Correspondent
This isn’t the first time the colourful and outspoken Mr Mahathir has resigned from his party – but at the age of 90 it will probably be his last.
Until recently he could count on strong support within Umno, the party which has governed Malaysia since independence. He previously resigned eight years ago in a move that helped to unseat Mr Najib’s predecessor.
But this time Mr Mahathir’s influence has been weakened, as Mr Najib has forced out of the party anyone who threatened his position, and has ensured a quick end to the investigation into the scandal.
Last year Malaysians were treated to the unexpected sight of Mr Mahathir joining anti-government street protests – which he rarely tolerated when he was in office.
But real power still lies within the ruling party, and now that he has left it, the veteran politician, for so long a dominant figure in public life, will have few opportunities left to challenge the government.
Mr Mahathir is Malaysia’s longest-serving prime minister, governing from 1981 until 2003, and remains highly influential.
Umno has led all of Malaysia’s ruling multi-ethnic coalitions since the country’s formation in 1957.
Earlier this month Dr Mahathir’s son, Mukhriz Mahathir, resigned his post as chief minister of Kedah state, saying he was ousted because of his criticism of Mr Najib.
The prime minister’s office said in a statement that Mukhriz Mahathir had lost majority support because he faced a lack of confidence and there were concerns about preparations for the party to retain Kedah in the 2018 elections.