BANGKOK, Aug 31 (Reuters) - Former Thai prime minister Thaksin Shinawatra is preparing documentation to draft a request for a royal pardon, but when it would be submitted was at his discretion, his lawyer said on Wednesday (Aug 30).

Thaksin returned to Thailand in a vaunted homecoming last week after spending 15 years abroad in self-exile. After landing in a private jet, he was transferred to prison to serve an eight-year sentence on charges of abuse of power and conflict of interest remnant from his days in power.

Hours after his arrival, Srettha Thavisin of the Shinawatra family-backed Pheu Thai party sailed through a parliamentary vote to become prime minister, fuelling speculation that Thaksin had struck a deal with his former enemies among the country's conservative and royalist military that ousted his government in 2006 and 2014.

Thaksin and the Pheu Thai party have denied this.

During his first night in Thailand, he was moved to a police hospital with chest pains and high blood pressure.

On Wednesday his lawyer Winyat Chartmontri told Reuters the former prime minister was preparing a request for a royal pardon.

"Thaksin is proceeding on his own and is currently in the preparatory stages of documentation and drafting the request," Winyat said, adding the legal team has not seen the request.

The timing of when the request will be submitted will be at Thaksin's own discretion, said Winyat.

Despite being away for 15 years, Thaksin still towers over Thai politics, known for populist policies like universal healthcare and cash handouts. Parties loyal to him have won every election in the last 20 years until 2023.

Anti-Thaksin groups have petitioned the corrections department to block any attempt to seek a pardon, citing that corruption cases should not receive clemency.