KUALA LUMPUR (Reuters) - Mahathir Mohamad, Malaysia's long-serving former prime minister, has resigned as an adviser to state-run oil company Petronas (PETR.UL), giving up a position he has held since he stepped down as leader in 2003, media reported on Monday.
Mahathir, 88, had resigned from the post effective December 1, the pro-government New Straits Times newspaper said. Mahathir's office did not immediately confirm the resignation and said it would issue a statement later in the day. There was no immediate response from Petronas.
The Star newspaper reported that Mahathir had decided to step down on his doctor's advice. Mahathir was admitted to hospital last month for a chest infection, although he has generally been in good health, frequently giving interviews and criticising the government from the sidelines.
The report coincided with the first day of the ruling United Malays National Organisation's (UMNO's) annual general assembly, which Mahathir was expected to attend.
Relations between Mahathir and current Prime Minister Najib Razak have grown tense in recent months, particularly over Najib's refusal to endorse Mahathir's son for an influential post as one of three UMNO vice presidents.
Mukhriz Mahathir lost his bid for the position in an internal party vote in October, with all three vice-president slots going to candidates backed by Najib.
Mahathir, still an influential power broker within UMNO, criticised Najib in an interview with Reuters just before the vote, saying he had only stayed in power after a poor election showing in May because of a lack of viable replacements.
The Petronas board reports directly to the prime minister.
Mahathir stepped down as prime minister after 22 years in power and was appointed as an adviser to Petronas - Malaysia's only Fortune 500 company - as well as national carmaker Proton, a position he still holds.
Those roles have been criticised by some observers, 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, has firmly established itself as a global energy player in recent years, spending around $5 billion last year to scoop up shale energy assets in Canada.
At home, it remains under pressure to fulfil 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)