The prime minister's press secretary Tengku Sharifuddin Tengku Ahmad has challenged former premier Dr Mahathir Mohamad to name who the new prime minister and deputy prime minister will be.

Failing which, Tengku Sharifuddin said Mahathir should cease his campaign against Prime Minister Najib Abdul Razak and quietly retire.

Tengku Sharifuddin said this is to put an end, once at for all, speculations that Mahathir was trying to install former deputy prime minister Muhyiddin Yassin in the top post in exchange for his son Mukhriz Mahathir to take over in future.

"There is a faster way for Mukhriz and his father to stop all the speculation that Mahathir is paving the way for his son.

"Mahathir just needs to announce who he wants to be prime minister and deputy prime minister.

"Why does he always avoid answering this one simple but essential question?

Why is he consistently evasive about his plans for Mukhriz's political future?" he said in a statement today.

Tengku Sharifuddin said this after Mukhriz threatened to sue him if he did not retract the claim that Mahathir was out to oust Prime Minister Najib Abdul Razak to pave the way for his son to become prime minister.

"If Mahathir wants his attempt to unseat the prime minister to have any credibility, he must at least give a straight answer on who the alternative candidate is.

"If he still refuses, he should stop this distraction for Malaysia and quietly retire with the grace people expect of a former statesman.

"It is the people, not one man, who under our constitution are entitled to democratically choose their government and prime minister at the next general election," he said.

Tengku Sharifuddin also stood his ground despite Mukhriz's threat to sue, insisting that Mukhriz had ambitions to become prime minister.

"Why then did he contest to be Umno vice-president in 2013?

“Everyone in Malaysian politics knows that only someone who wants to be Umno president, and then by default prime minister, would stand to be vice-president.

"It is a key step on the ladder to the top," he said.

Tengku Sharifuddin suggested that Mukhriz may only now be suggesting he did not want to be prime minister because he lost in the contest.

"Or maybe because he was told to say this by his father, Mahathir Mohamad.

"But we all know that he or his father did want him to climb the ladder, because he stood for election as vice-president," he said.