The search was on for Thailand's former prime minister Yingluck Shinawatra on Saturday after she failed to appear for a court verdict in a criminal case that could send her to prison for 10 years.

Yingluck's whereabouts were not immediately known and her absence drove speculation she fled the country.

An official of Yingluck's Pheu Thai party, who is close to the Shinawatra family, told The Associated Press she was no longer in Thailand. The official gave no other details and declined to be identified because of the sensitivity of the subject.

Yingluck, 50, who became Thailand's first female prime minister when her party swept elections in 2011, is accused of negligence in overseeing a money-losing rice subsidy programme. She pleaded innocent and decried the charges as politically motivated.

A verdict had been expected on Friday as thousands of Yingluck supporters gathered outside the court and thousands of police stood guard. But she never appeared and a judge read out a statement saying her lawyers had informed the court she could not attend because of an earache.

The judge said the court did not believe the excuse, however, because no official medical verification was provided. He said a warrant would be issued for her arrest and announced the trial would be postponed until September 27.

Norrawit Larlaeng, Yingluck's lawyer, said he had no details on her whereabouts. "I was told this morning that she was ill, that she had vertigo, that she felt dizzy, so I requested the postponement ... That's all I have to say."

Prime Minister Prayuth Chan-ocha, the military chief who engineered the 2014 overthrow of Yingluck's government, said the military government was "looking for her".

"If she's not guilty, she should stay and fight the case," Prayuth said. "If she's not here, what does that tell you? Will she still say that she didn't get justice?"

Defence Minister Prawit Wongsuwan said security officials monitoring Yingluck had not seen her leave her Bangkok home in the last two days.

The trial is the latest chapter in a decade-long struggle by the nation's elite minority to crush the powerful political machine founded by Yingluck's brother, Thaksin Shinawatra, who was toppled in a 2006 coup. Thaksin has lived in Dubai since fleeing a corruption conviction.

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Prawit Pongkunnut, a 55-year-old rice farmer from the northeastern city of Nakhon Ratchasima, said he came to the court with 10 other farmers to show solidarity with Yingluck.

"We're here to give her moral support because she truly cared and helped us out," Prawit said.

Yingluck's former commerce minister was jailed in a related case for 42 years on Friday.

"She has definitely left Thailand," one source, who is also a member of her Puea Thai party, told Reuters news agency. The sources did not say where she had gone.

Cambodian immigration police said she had not entered their country.

If Yingluck has fled it would disappoint her supporters and make her opponents feel vindicated, said Thitinan Pongsudhirak, director of the Institute of Security and International Studies at Chulalongkorn University.

"It does not help with Thailand's division and polarisation," he said.