After his election defeat, Najib stepped down as chairman of Barisan Nasional, the coalition that has ruled the country since independence from Britain in 1957, and as president of the United Malays National Organisation, the largest party within that coalition.

Malaysia's Immigration Department also slapped a travel ban on the scandal-ridden former prime minister and his wife Rosmah Mansor.

The ban on Najib and his wife leaving the country came shortly after he announced plans for a short holiday. A leaked flight manifesto showed Najib and Rosmah were due to leave on a private jet to Jakarta, fuelling rumours he was fleeing the country to escape possible prosecution over the 1MDB scandal.

Mahathir, now the world's oldest elected leader at 92, has said his government would not seek revenge over the 1MDB fiasco, but that Najib would have to face the court if he was found to be responsible.

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The fund was started by Najib when he took power in 2009, but it accumulated billions in debts and is being investigated in the US and several other countries. US investigators said Najib's associates looted $US4.5 billion from the fund, of which $US681 million landed in Najib's bank account and some $US30 million used to buy jewellery for his wife.

Najib denies any wrongdoing. He sacked critics in his government including an attorney-general and a deputy prime minister and muzzled the media. The new Attorney-General Mohamed Apandi Ali cleared Najib in 2016, saying the money was a donation from the Saudi royal family and that most of it was returned.

Mahathir, who is due to name his cabinet later on Saturday, has indicated that Attorney-General Mohamed Apandi would be axed for hiding evidence of wrongdoing.