Yingluck, 50, was due on Friday morning to arrive at the Supreme Court for the ruling in her trial for criminal negligence that could have seen her jailed for 10 years.
But she did not show up, staging a vanishing act that wrote a dramatic closing chapter to the 16-year political saga of her mega-rich Shinawatra family.
Speculation swirled on Saturday on the whereabouts of Thailand's first female prime minister - and her possible escape route.
The junta source, who is well-placed in the security hierarchy, gave a detailed description of her escape, saying she took a private jet from Thailand to Singapore and onto Dubai, the base of Shinawatra family patriarch Thaksin, who is Yingluck's older brother.
"Thaksin has long prepared escape plan for his sister... he would not allow his sister to spend even a single day in prison," the source added, requesting anonymity.
"But Dubai is not Yingluck's final destination," the source said, adding she may be aiming "to claim asylum in Britain".
Thaksin, who once owned Manchester City football club, owns property in London and spends significant amounts of time in the city.
The Shinawatra's political network remained tight-lipped on Saturday in a media blackout that only served to heighten speculation over her dash from Thailand and the likelihood of a possible deal with the junta to allow her to leave.
A senior source inside the family's Pheu Thai party, also requesting anonymity, on Saturday told AFP Yingluck had fled the country for Dubai a few days before the ruling.
The Shinawatra political dynasty emerged in 2001 with a series of groundbreaking welfare schemes that won them votes and the loyalty of the rural poor.
But their popularity rattled Thailand's royalist, army-aligned elite, who battered successive governments linked to the clan with coups, court cases and protests.
Yingluck's government was toppled by a coup in 2014 and she was put on trial over negligence linked to a costly rice subsidy that propped up her rural political base.
Thaksin, Yingluck's elder brother, has been based partly in Dubai since he fled Thailand in 2008 to avoid jail for a corruption conviction. He was toppled from power by a 2006 coup.
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Houdini act
Thai newspapers reported that Yingluck fled through a land border to Cambodia, flew to Singapore and on to Dubai, perhaps two days before her court date.
It was a curveball that appeared to surprise even her family -- an elder brother and sister waited at the court for her arrival alongside thousands of supporters.
Shinawatra loyalists expressed sympathy with her shock move, saying the ruling would have been predetermined as her case was politically motivated.
"If she has fled abroad it is because this set of judges are appointed by the military and do not come from a democratic system," Surachet Chaikosol, 59, a 'Red Shirt' activist told AFP.
"I am glad she will not suffer in jail."
Thai junta chief Prayut Chan-O-Cha on Friday denied knowledge of Yingluck's whereabouts and expressed surprise at her no-show "as she always insisted that she would fight the case".
Akanat Promphan, a key member of the anti-Shinawatra protests that presaged the 2014 coup, said Yingluck's flight reflected badly on her chances in court.
"Obviously she wasn't confident of her innocence despite lying to the public that she would stand trial to the bitter end," he said.
But analysts say Yingluck, who was closely monitored by Thai security services, most likely cut a deal with the junta to exit the country.
Had she been jailed, Yingluck's plight could have stirred anger and unrest from her large support base.
The military is desperate to avoid instability as it digs in for a long stay in Thai politics.
It says elections will be held next year.
But critics say any government that emerges from elections held under a new military-drafted constitution will be straitjacketed, while the economy and society are likely to atrophy under a 20-year junta 'reform' strategy.
