KUALA LUMPUR, March 31 (Reuters) - Malaysia's deputy premier Najib Razak is likely to be appointed Prime Minister on Friday, a high-level government source said on Tuesday.

Najib is expected to be sworn in as Malaysia's sixth prime minister a day after outgoing premier Abdullah Ahmad Badawi gets the king's consent to step down.

"He will be sworn in most likely on Friday morning," the source, who declined to be identified, told Reuters.

The power to appoint Malaysia's prime ministers lies with the king, a constitutional monarch. Malaysia has a system of rotating its kings from among its nine sultans.

Najib, a British trained economist, is the son of the country's second Prime Minister Abdul Razak Hussein.