A Thai court will deliver a verdict on former prime minister Thaksin Shinawatra in a royal insult case on August 22, his lawyer said on Wednesday.

"We are confident that we will receive justice," Winyat Chartmontree said.

Thaksin, suspended Prime Minister Paetongtarn Shinawatra's father, remains influential in Thai politics.

He testified in court on Wednesday, seeking to defend himself against royal defamation charges in a watershed case for his faltering political dynasty.

Thaksin faces up to 15 years in prison if he is convicted in the closed-door trial in Bangkok, where he stands accused of breaching strict lese majeste laws that shield Thailand's royal family from abuse and criticism.

For the past quarter-century, the 75-year-old telecoms magnate has been a defining figure of Thai politics, founding a political clan which has jousted with the traditional pro-royal, pro-military elite.

But his prosecution – combined with the suspension of his daughter, Prime Minister Paetongtarn Shinawatra, this month – represents a dramatic waning of their family's political fortune, analysts say.

The prosecution's case revolves around remarks Thaksin made to South Korean media a decade ago. A verdict is not expected for several weeks.

Winyat said he gave testimony in the morning "and will continue throughout the rest of the day".

Around 50 Thaksin supporters gathered at the courthouse wearing shirts the red colour of his political movement and emblazoned with a portrait of his face. (Reuters/AFP)