Jailed former Thai premier Thaksin Shinawatra will be freed on Sunday, the kingdom's prime minister has said, just six months after returning from 15 years of self-imposed exile.

The controversial billionaire, twice elected premier and ousted in a 2006 military coup, was jailed for eight years on graft and abuse-of-power charges upon his return to Thailand in August, but had his sentence cut to one year within days by King Maha Vajiralongkorn.

Speaking to reporters on Saturday before boarding a flight, Prime Minister Srettha Thavisin said Thaksin's release would be "on the 18th" and handled "according to the rule of law".

It comes after Justice Minister Tawee Sodsong said earlier this week that Thaksin, 74, would be among 930 prisoners granted early release.

His homecoming coincided with his Pheu Thai party returning to government in alliance with pro-military parties, leading many to conclude that an agreement had been struck to cut his jail time.

The rumours grew stronger when he was transferred to a police hospital within hours of being sentenced because of his poor health, and it is not clear that he has spent any time in a prison cell.

Local media reported that Thaksin had been suffering from chest tightness and high blood pressure when he was admitted to hospital, and his family said he had undergone two operations in the following months.