BANGKOK/KHON KAEN (REUTERS, BLOOMBERG) - Hundreds of supporters of ousted former Thai prime minister Yingluck began to gather outside Bangkok's Supreme Court early on Friday (Aug 25), hours before the court was due to rule on a negligence case against her in which she faces up to 10 years in prison.

The long-awaited verdict could inflame tension in the South-east Asian country and have far-reaching implications in the politically divided kingdom.

A ruling is also due in a related case against Boonsong Teriyapirom, who was commerce minister in Yingluck's government.

Bangkok's metropolitan police said around 4,000 police were deployed at the court and checkpoints had been set up.

A rice subsidy programme - a flagship policy of Yingluck's administration - saw her government buy farmers' crops at prices up to 50 per cent higher than market prices. The policy was popular with farmers but left Thailand with huge rice stockpiles and caused US$8 billion (S$10.8 billion) in losses.

Yingluck, who has pleaded not guilty to the negligence charges against her, has said she was in charge of only coming up with the policy but not the day-to-day management of the scheme. Her administration was removed in a 2014 military coup.

"A 'guilty' or 'not guilty' verdict will infuriate one side or the other of Thai political society," said Paul Chambers, research director at the Institute of South East Asian Affairs in Chiang Mai.

"Thailand remains incredibly divided over the Shinawatras, 16 years after Thaksin was first elected."

Investors will be watching the outcome of the trial closely.

The benchmark SET index of stocks has climbed 2 per cent this year, one of the worst performers in Asia, weighed down in part by a climb in bad loans in Thailand. Bonds have proved a bigger draw for foreign investors, helping to make the baht the region's top-performing currency in 2017.

"An acquittal would relieve any concern about follow-up violence," said Win Udomrachtavanich, chairman of Ktb Securities (Thailand) in Bangkok.

"A verdict against Yingluck would add downside risk since political uncertainty will climb. It's hard to predict what would happen next in this scenario."

The junta clamped down on political activity after seizing power three years ago following a period of unrest, pledging to restore stability. The current stretch of military rule is one of the longest since the 1970s, in a country with a history of coups since the end of absolute monarchy in 1932.

In the north-eastern province Khon Kaen, a Shinawatra stronghold, a leader of the "red shirt" political movement that supports Yingluck said her supporters felt frustrated. "Some people may want to demonstrate publicly to show their unhappiness about how Yingluck is being treated," he said.

Shinawatra's Puea Thai Party has said it does not support acts of violence and urged supporters to gather peacefully.

Some supporters outside the court in Bangkok held roses while others wore white gloves with the word "love" on them.

Bangon Saeliang, 56, a market vendor from Bangkok, said she turned up to support Yingluck because she felt the former prime minister had done nothing wrong. "Farmers benefited from the scheme," she said.

Before giving a verdict in Yingluck's case, the court will rule in the case of Yingluck's former commerce minister Boonsong Teriyapirom, who is accused of falsifying government-to-government rice deals between Thailand and China in 2013.

The Shinawatra family has dominated Thai politics for more than 15 years and their parties have won every general election since 2001, but it is at the heart of a bitter and bloody power struggle in Thailand.

Yingluck's brother, former prime minister Thaksin Shinawatra, was ousted in a 2006 coup. Supporters of the Shinawatras accuse the ruling generals of political persecution.

The military government has acknowledged it wants to maintain permanent influence over future elected governments, partly through a new Constitution that took effect earlier this year.

Prime Minister Prayuth Chan-ocha, who led the 2014 coup against Yingluck's government, has said the coup was to end political turmoil in the country. He has promised that an election will be held next year.

Regardless of whether she is found guilty or innocent, Yingluck will not be able to run in that election because she was banned from politics for five years in 2015 by the junta's legislature for alleged graft in the rice-purchasing programme.

If Yingluck is found guilty, she has 30 days to appeal and is expected to post bail, avoiding any immediate prison time.