Anti-graft agents are investigating how billions of dollars went missing from the 1Malaysia Development Board state fund, known as 1MDB, which Najib founded and chaired.

Najib has been accused of pocketing $700m from the state fund and faces 32 charges in relation to 1MDB. He denies all wrongdoing.

His wife Rosmah Mansor was charged with activities using illegal proceeds and failing to declare income tax, both covered under Malaysian anti-money laundering law.

She pleaded not guilty to all charges in a Kuala Lumpur court last week.

It is unclear if the charges against the former first lady relate to 1MDB, but her arrest came after three rounds of questioning by anti-graft agents over the state fund scandal.

If convicted, she would face between 5 and 15 years in prison and possible fines for each charge.

The 1MDB scandal erupted in 2015 after media reported that hundreds of millions of dollars from the fund were diverted into Najib personal accounts.

The fund was intended to help turn the Malaysian capital Kuala Lumpur into a financial hub and boost the economy through strategic investments.

In June Malaysian police seized goods and cash worth up to $273m from properties linked to Najib as part of the investigation.