KUALA LUMPUR (Reuters) - Mahathir Mohamad, Malaysia's long-serving former prime minister, said on Monday he had resigned as an adviser to state-run oil company Petronas (PETR.UL), ending an almost decade-long tenure in the energy major that is Malaysia's only Fortune 500 company.
The 88-year-old Mahathir told reporters he resigned, effective December 1, because doctors had advised him to "slow down".
His departure, however, comes as relations with Prime Minister Najib Razak have grown strained in recent months, particularly over Najib's refusal to endorse Mahathir's son for an influential ruling party post. The Petronas board reports directly to the prime minister.
Petronas declined to comment and it was not immediately clear what effect Mahathir's absence would have on the company. Mahathir's office said he had informed Najib of his resignation in a hand-written note, but gave no further details.
Mahathir was admitted to hospital last month for a chest infection, although he has generally been in good health, frequently giving interviews and criticizing the government from the sidelines.
The news of Mahathir's resignation coincided with the first day of the ruling United Malays National Organisation's (UMNO's) annual general assembly, which Mahathir was expected to attend.
Mahathir's son, Mukhriz, lost his bid for the influential post of one of three UMNO vice presidents in a party vote in October. All posts were filled by candidates backed by Najib.
Mahathir, who still holds political clout within UMNO, criticized Najib just before the vote, telling Reuters in an interview that the prime minister had only stayed in power after a poor election showing in May because of a lack of viable replacements.
Mahathir stepped down as prime minister after 22 years in power and was appointed as an adviser to Petronas and national carmaker Proton, which is owned by DRB-Hicom (KLS:DRBHCOM).
Mahathir would retain his several other advisory positions, his office said.
His advisory roles at Petronas and Proton have been criticized by some opposition politicians, who say Mahathir has used his influence to ensure contracts go to allies and business associates. Mahathir has denied such accusations.
Petronas, widely seen as one of Malaysia's most efficient state firms, is a global energy player which last year spent around $5 billion on shale energy assets in Canada.
At home, it remains under pressure to fulfill a social and political role. It is by far the government's biggest revenue source and a pillar of a longstanding national policy to promote a bigger economic role for majority ethnic Malays.
(Reporting by Stuart Grudgings; Editing by Ron Popeski and Miral Fahmy)