Allegations that the RM2.6 billion donation in Prime Minister Najib Abdul Razak's personal accounts came from 1MDB is a lie by former premier Dr Mahathir Mohamad, said a Najib aide.

"The allegation that the funds were from 1MDB is completely false.

"This is a lie that Mahathir has unsuccessfully used to try and unseat a democratically-elected government," Najib's press secretary Tengku Sariffuddin Tengku Ahmad said in a statement today.

Tengku Sariffuddin then took a swipe at Mahathir's family.

"He (Mahathir) placed saving the political career of his son, Mukhriz Mahathir, above the national interest."

The premier's press liason was responding to a Wall Street Journal report today claiming that funds received by Najib had originated from 1MDB.

It alleged that part of the US$3.5 billion paid by 1MDB to a fake Aabar firm had ended up in another firm which transfered the RM2.6 billion into Najib's accounts.

"The prime minister has consistently maintained that the funds were a donation from the Royal Family of Saudi Arabia," Tengku Sariffuddin insisted.

He added that this was backed up by Saudi Foreigner Minister Adel al-Jubeir who said he was aware of the donation.

Najib has accused the WSJ of being part of a plot to topple him.

Attorney-general Mohamed Apandi Ali has cleared the premier of any criminal misconduct in the RM2.6 billion donation case.