Thailand has tightened border controls after former PM Yingluck Shinawatra failed to show up for the verdict in her trial over a rice subsidy scheme.
Deputy PM Prawit Wongsuwan said it was possible she had already fled.
Lawyers for Ms Yingluck, who is charged with negligence, said she was unable to attend court because she was ill.
But the Supreme Court issued an arrest warrant for her, confiscated her bail of $900,000 (£703,000) and delayed the verdict to 27 September.
Ms Yingluck has denied any wrongdoing in the scheme which cost Thailand billions of dollars. If found guilty, she could be jailed for up to 10 years and permanently banned from politics.
On Friday, Prime Minister Prayuth Chan-ocha said all routes out of the country were being closely monitored.
"I just learned that she did not show up [at court]," he told reporters. "I have ordered border checkpoints to be stepped up."
Mr Prawit initially said he had no information on Ms Yingluck's whereabouts but as he left a meeting in Bangkok he said: "It is possible that she has fled already."
Earlier, Ms Yingluck's lawyer requested a delay in the ruling, telling the court that she had vertigo and a severe headache and was unable to attend.
But an official Supreme Court statement said it did not believe she was sick as there was no medical certificate and that the claimed sickness was not severe enough that she could not travel to court.
"Such behaviour convincingly shows that she is a flight risk. As a result, the court has issued an arrest warrant and confiscated the posted bail money," the statement said.
Asked by the BBC whether she was still in the country, Ms Yingluck's lawyer Norrawit Larlaeng said: "I don't know. I don't know."
Dilemma over Yingluck's fate
Jonathan Head, BBC News, South East Asia correspondent
The possible fate of Thailand's first female prime minister had generated a great deal of debate and anxiety in the days leading up to this court hearing.
The military government had tried to prevent or discourage Ms Yingluck's supporters from coming to the Supreme Court in Bangkok, for fear of an angry reaction should she be convicted and imprisoned.
Her failure to turn up for the verdict now presents the authorities with another dilemma - sending police to arrest her might also provoke unrest.
Ms Yingluck's brother, controversial former prime minister Thaksin Shinawatra, lives in exile after fleeing the country in 2008 to evade a jail term for corruption.
Friday's turn of events took many by surprise, including the hundreds of people who turned up outside the Supreme Court in Bangkok to support Ms Yingluck.
BBC Thai reporter Nanchanok Wongsamut, who was in court, said the announcement prompted shocked reactions in the room, and then a flurry of activity as journalists ran out to report the news.
But some supporters outside the court expressed understanding.
"The Thai prime minister has done her best, she has sacrificed a lot," said Seksan Chalitaporn, 64. "Now the people have to fight for themselves."
Ms Yingluck, who became Thailand's first female prime minister in 2011, was impeached in 2015 over the rice scheme by a military-backed legislature, which then brought the legal case.
The scheme, part of Ms Yingluck's election campaign platform, launched shortly after she took office.
It was aimed at boosting farmers' incomes and alleviating rural poverty, and saw the government paying farmers nearly twice the market rate for their crop.
But it hit Thailand's rice exports hard, leading to a loss of at least $8bn and huge stockpiles of rice which the government could not sell.
Though popular with her rural voter base, opponents said the scheme was too expensive and open to corruption.
During her trial, Ms Yingluck had argued she was not responsible for the day-to-day running of the scheme. She has insisted she is a victim of political persecution.
Timeline of Yingluck's rice scheme controversy
May 2011- Yingluck Shinawatra is elected PM, and shortly afterwards begins rolling out her rice subsidy scheme.
January 2014 - Thailand's anti-corruption authorities investigate Ms Yingluck in connection to the scheme.
May 2014 - She is forced to step down from her post after Thailand's constitutional court finds her guilty of abuse of power in another case. Weeks later the military ousts what remains of her government.
January 2015 - An army-backed legislature impeaches Ms Yingluck for corruption over her role in the rice scheme, which effectively bans her from politics for five years. It also launches legal proceedings against her.
August 2017 - Ms Yingluck fails to appear at court for the verdict, claiming ill health.
Ms Yingluck's time in office was overshadowed by controversy as well as strong political opposition.
The youngest sister of Mr Thaksin, she was seen by her opponents as a proxy for her brother, who was controversially ousted by the military in 2006.
Both siblings remain popular among the rural poor, but are hated by an urban and middle-class elite.
Their Pheu Thai party has - under various different names - won every election in Thailand since 2001.