Thailand's former prime minister, facing a possible 10-year jail term in a court case her supporters say was politically motivated, has fled the country, a top member of her party said.

Yingluck Shinawatra's whereabouts were not immediately known, but local media cited anonymous officials as saying she travelled by land to Cambodia, then flew to Dubai to join her brother, former Manchester City FC owner Thaksin Shinawatra - another exiled former premier whose government, like hers, was toppled in a military coup.

A senior member of Ms Yingluck's Pheu Thai party, who is close to the Shinawatra family, said she was no longer in Thailand.

The official gave no other details, and declined to be identified because of the sensitivity of the subject.

Ms Yingluck, who became Thailand's first female prime minister when her party swept to election victory in 2011, is accused of negligence in overseeing a money-losing rice subsidy programme. She has denied the charges, calling them politically motivated.

A verdict had been expected on Friday, as thousands of Yingluck supporters gathered outside the court in Bangkok and thousands of police stood guard.

But Ms Yingluck 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 because no official medical verification was provided. He said a warrant would be issued for her arrest and postponed the trial until September 27.

Ms Yingluck's lawyer Norrawit Larlaeng confirmed a warrant had been issued, but 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," he said.

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

"If she's not guilty she should stay and fight the case," Mr 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 forces had not allowed Ms Yingluck to leave and were checking possible routes she may have used.

He said security officials monitoring Ms 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 Ms Yingluck's brother Thaksin, who was toppled in a 2006 coup.