A Bersatu supreme council member has refuted Dr Mahathir Mohamad's assertion that the party was now supporting former premier Najib Abdul Razak and other alleged "kleptocrats".

Muhammad Faiz Na'aman said this in reply to a letter Mahathir had sent him.

In the letter, the disputed Bersatu chairperson said he was saddened that they had jointly "vowed to topple Najib's kleptocrat government" but the party was now siding with the former prime minister instead.

Faiz said it was true that he vowed to fight corruption before the 14th general election, and he still holds to this principle.

However, he said that with the cases now in court, they have done all that they possibly can.

"That which determines if someone is truly guilty or innocent is the courts, not political parties, not the prime minister, and not the media or social media.

"That is the rule of law that Tun (Mahathir) himself always promotes," he said in a statement.

Najib is on trial for corruption and money laundering in relation to the SRC International and 1MDB cases.

Mahathir, however, claimed that Prime Minister Muhyiddin Yassin would not allow Najib to be convicted to retain a fragile hold on the government.

Faiz expressed confidence that the courts were independent and free from political interference, and that solid evidence must be presented to support any claims that any judges are corrupt.

"And it is not true if Tun's side claims that those who support the Perikatan Nasional government led by Muhyiddin is akin to supporting Najib or those whom Tun considers kleptocrats," he added.

Faiz said the formation of PN was the result of Mahathir's surprise resignation in February.

He added that it was also a manifestation of the desires of the majority, especially Malays, who were tired of PKR leader Anwar Ibrahim's succession drama and were also against DAP.

Mahathir resigned in February after refusing to go along with Muhyiddin's plan for Bersatu to form a new government with Umno, BN and PAS.