The ban comes after Najib Razak said that he and his wife were planning to go on an overseas holiday on May 12.
Earlier this week, Najib Razak’s long-ruling Barisan Nasional coalition suffered a shock electoral defeat.
Najib Razak has been accused of diverting $700 million from a state investment fund in 2015, but has since been cleared by the authorities.
However, Malaysia’s new PM Mahathir Mohamad, who at 92 became the world’s oldest elected leader when he was sworn in on May 10, has said that Najib Razak could face a fresh investigation if sufficient evidence supports it.
Mahathir Mohamad has said that investigations will take place into alleged corruption in Malaysia, including the case involving the state investment fund.
He stood down as prime minister 15 years ago, but came out of retirement and defected to the opposition to take on and beat former protégé Najib Razak.
Mahathir Mohamad appears to be making good on a promise, made during his election, to release imprisoned former political leader Anwar Ibrahim.
On May 12, Anwar Ibrahim’s daughter, Nurul Izzah, confirmed reports that her father will “receive a full royal pardon” and is due to be released on May 15.
Anwar Ibrahim was once Mahathir Mohamad’s deputy, but was jailed on widely-criticized charges of sodomy and corruption amid bitter tensions between the two men.
Najib Razak tweeted that he had been informed by the immigration authorities that he and his family would not be allowed to travel abroad.
The former prime minister gave no reasons for the authorities’ decision, but said he would abide by it.
Najib Razak, 64, has also announced he is quitting as chairman of the Barisan Nasional coalition and as president of his United Malay National Organization party.
He had said earlier that he and his wife Rosmah Mansor would go on a holiday on May 12.
It is believed they intended to fly to Indonesian capital, Jakarta.
Najib Razak had faced accusations of corruption and mismanagement over the state investment fund 1Malaysia Development Berhad (1MDB).
The scandal has gripped the country for years.
1MDB, set up by Najib Razak in 2009, was meant to turn Kuala Lumpur into a financial hub and boost the economy through strategic investments.
However, the fund started to attract negative attention in early 2015 after it missed payments for some of the $11 billion it owed to banks and bondholders.
Then the Wall Street Journal reported it had seen a paper trail that allegedly traced close to $700 million from the fund to Najib Razak’s personal bank accounts.
Najib Razak has consistently denied taking money from 1MDB or any public funds.
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