Prime Minister Dr Mahathir Mohamad has confirmed that he prevented former premier Najib Abdul Razak from leaving the country.

"It is true that I prevented Najib from leaving the country," he said at a press conference at the Bersatu headquarters in Petaling Jaya today.

When asked if Najib was blacklisted by the Immigration Department because of pending investigations, Mahathir replied: "There are a lot of complaints against him, all of which have to be investigated."

"If some of the complaints are valid...We have to act quickly because we don’t want to be saddled with extradition from other countries," he added. 

On how soon the 1MDB investigations can be launched, the prime minister said various countries need to be contacted, and numerous reports need to be sifted through.

The countries they intend to contact include the US, Switzerland, Singapore and Luxembourg. 

These are the countries which have opened investigations into 1MDB. 

Najib has been implicated in the 1MDB scandal, but has denied any wrongdoing in the matter. 

The blacklisting of the former prime minister and his wife Rosmah Mansor came amid a leaked flight plan yesterday, which showed that they planned to travel from Subang Airport to Jakarta's Halim Perdanakusuma International Airport at 10am today.