BANGKOK, July 16 -- Former Thai prime minister Thaksin Shinawatra is set to testify in court on todayseeking 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 shielding 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 dynasty 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...