Mahathir won the election after his coalition Pakatan Harapan secured 113 seats out of the 222 seats at the parliament's lowerhouse, unseating the Barisna Nasional coalition, which has run the country for more than 60 years and of which Mahathir used to serve as the chairman. The 92-year-old defected from Najib's party in 2016.
Mahathir undoubtedly helped to carry the opposition to victory by appealing to voters sick of Najib's corruption-plagued rule, but his comeback has also been controversial.
"Because no party has gotten a simple majority, therefore the king will be making a decision as to who will be the prime minister", Najib said.
The statement added that King Muhammad strongly support and respect democracy and that the reports did not reflect the truth.
There was a sense of catharsis after the opposition alliance, boosted by the presence of political titan Mahathir, finally ousted a coalition that has for years been accused of massive corruption, fanning racial tensions in the multi-ethnic country and economic mismanagement.
But official reasons for the delay have not been made clear. He's also suspected of trying to manipulate the popular vote after backing legislation to redraw the electoral map.
He said people's livelihoods had been affected under Najib's rule due to the rising cost of living, a major issue in the election campaign.
More broadly, Mr. Najib has been embroiled in a scandal involving billions of dollars that disappeared from a government investment fund that he once headed, 1Malaysia Development Berhad.
Mahathir has said he would clear the way for criminal prosecution of Najib. "You don't have all the institutions which are created to give form to the government of this country".