Mahathir Mohamad said he had stepped down as an adviser of Petronas, Malaysia's state-run oil firm and the only company in the country that has made it to the Fortune 500. Mahathir is the longest-serving prime minister in the Southeast Asian country. His resignation marked an end to a nearly decade-long tenure with the oil firm, Reuters reported.

Mahathir, 88 years old, told reporters that his resignation which took effect on December 1, was prompted by his doctors' advice to slow down. However, a Reuters report said his resignation comes at a time when his relationship with Prime Minister Najib Razak had become strained. The thaw in the relationship between the two leaders was over the refusal of Najib to endorse the son of Mahathir for a post in the country's ruling party, the report said. The board of directors of state-run Petronas directly reports to Malaysia's prime minister.

The office of Mohamad said he had told Najib of his departure through a handwritten note but did not elaborate further. Petronas did not also comment on the resignation when asked by Reuters.

Last month, Mohamad was confined in the hospital due to a chest infection, but Reuters reported that he has generally been in good health. He frequently granted interviews and aired his criticisms of the government from the sidelines, the report added.

Mohamad served as Malaysia's prime minister for 22 years. When he stepped down, he was appointed as an adviser to Petronas as well as to Proton, the Malaysia's national carmaker. DRB-Hicom owns Proton. Mohamad's office, however, said he would continue to serve in his other advisory positions.

Petronas is seen as one of the most efficient state companies in Malaysia, Reuters reported. It is a major energy player in the world. It spent about USD 5 billion in Canadian shale energy assets last year, the report added.