Najib, his wife and key allies have been been hit with a flurry of charges since his surprise election loss earlier this year as public anger crescendoed over a series of graft scandals.

Most of the charges have stemmed from the 1MDB scandal where top officials allegedly looted billions from a government fund to go on a worldwide spending spree.

But the latest probe centres on a controversial $1.2 billion deal signed sixteen years ago when Najib was defence minister to purchase two Scorpene-class submarines.

"Najib is being probed with regards to the purchase of the two Scorpene-class submarines," an official inside the Malaysian Anti-Corruption Commission (MACC) familiar with the ongoing probe, told AFP.

"On Monday, MACC officials grilled Najib for four hours over the deal," the officer added, requesting anonymity.

The submarine deal has long been mired in controversy.

French submarine maker -- then called DCNS -- is alleged to have paid more than 114 million euros ($134 million) in kickbacks to a shell company linked to Abdul Razak Baginda, a close Najib associate who brokered the deal.

Razak Baginda's Mongolian mistress Altantuya Shaariibuu, who was said to have demanded a cut for translating during negotiations, was shot dead and her body blown up with military-grade plastic explosives near Kuala Lumpur in 2006.

The case sank off the radar after a Malaysian court in 2008 cleared Razak Baginda of abetting the murder, sparking allegations of a huge cover-up to protect Najib.

But Najib's election defeat has reignited interest from Malaysia's anti-graft body.

The MACC official said Razak Baginda may also be hauled up to assist in their investigation of the submarine purchase.

A French investigation since 2010 has already led to four French executives involved in the deal being charged as well as an open criminal case against Razak Baginda.

Both Razak Baginda and Najib's government have denied any wrongdoing as well as the four French executives.

DCNS has since changed its name to Naval Group. It is part owned by the French state and by French multinational giant Thales.

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