Forex

The wife of deposed Thai prime minister Thaksin Shinawatra has been sentenced to three years in prison for evading millions of US dollars in taxes, the first conviction against his family since he was toppled in a 2006 coup.

Pojaman Shinawatra and her brother were given three years in prison, while her secretary received a two-year sentence.

"The actions by the three defendants are serious violations of the law. The court has decided that the three defendants were guilty of tax fraud," Judge Pramote Pipatpramote said.

All three were released on bail of five million baht each ($A158,950) pending appeal.

The family's spokesman immediately announced an appeal against the verdict.

"We will appeal within 30 days... We will wait for the decision from the highest court. We respect the decision of the highest court," Phongthep Thepkanjana said.

The case is one of a dozen corruption claims against Thaksin, his family and his political allies currently working their way through the legal system.

Analysts agreed the verdict signals a tougher line by the courts against the former premier.

"It's very significant for both of them. Nobody doubts this was a family matter. It will reflect on all of them," Bangkok-based analyst and Thaksin biographer Chris Baker said.

"I don't think we need to take the (corruption) cases as a team effort, I think each one will be decided on its merits," he said.

"The courts here are often intimidated by people in power or who were in power and they are reluctant to convict. In this case that doesn't seem to be operating."

"It is a harbinger of things to come. It is very foreboding for Thaksin's legal process... The verdicts have gone in one direction so far," said Thitinan Pongsudhirak, a political analyst at Chulalongkorn University in Bangkok.

Pojaman, 51, and the two other defendants were convicted of colluding to evade tax worth 546 million baht ($A17,202,130) in a 1997 transfer of shares in the family's Shinawatra Computer and Communication company, which later became Thailand's telecom giant Shin Corp.

The defendants denied the charges, insisting the shares were a gift so would be tax-free.

Baker said it was an "exceptionally damaging" judgement which the Shinawatra family were likely to fight as long as possible.

"It will go on - they will use every possible means under the law, but their ability to use other methods to influence any decision now is highly limited," he said.

Thaksin and Pojaman are due to appear before the Supreme Court on further corruption charges on August 11 but have received special permission to travel abroad in the meantime.

Thaksin leaves Thailand late on Thursday to give a speech on leadership in the southern Japanese city of Fukuoka before meeting Pojaman in Beijing for the Olympics opening ceremony.

Pojaman smiled as she left the court wearing sunglasses and a sombre grey suit, flanked by Thaksin and their three children, and greeted by hundreds of supporters carrying red roses.

Some 100 other supporters had filled the courtroom as the verdict - broadcast live on nationwide television - was read out.

The trial also attracted a huge security presence, with 200 police and guards surrounding the court, some wearing riot gear.

Pojaman, who rarely speaks in public, is widely seen as an important partner in Thaksin's vast political and business interests, which include being owner of English Premier League football club Manchester City.

While Thaksin stayed abroad for months following the coup, Pojaman shuttled in and out of the country to manage her husband's affairs in Bangkok between travelling to meet him in various spots around the world.