The controversy centres around whether Thaksin, who currently lives in self-exile, should have paid taxes on the sale of his telecoms company Shin Corp to Singapore's Temasek Holdings in 2006.
The furore over the deal, which netted the Shinawatra family a $1.9 billion windfall, was a lightning rod for opposition to his government.
Protests culminated in a coup that booted him from office and sparked years of debilitating political infighting between his supporters and opponents.
On Tuesday tax officials accompanied by police posted the bill on Thaksin's former home in Bangkok, although he has not set foot there for nearly a decade to avoid jail for a graft conviction that he insists was politically motivated.
Denying it was an act of political theatre, Thai junta leader Prayut Chan-O-Cha said tax officials "posted the summons at his house like they would in every other case."
Prayut is a former army chief who became prime minister after leading a 2014 coup against Thaksin's sister Yingluck.
"This is about a violation of the law and has nothing to do with reconciliation," he told reporters, referring to a recent junta-backed push to launch talks between Thailand's political factions.
Thaksin's representatives hit back, saying there was no legal requirement at the time for him to pay taxes on the telecoms company sale.
"Thaksin was not supposed to pay taxes from the very beginning and he will file his appeal within three days," his legal advisor Noppadon Pattama told reporters on Tuesday.
Anti-Shinawatra factions who agitated for the 2014 army takeover have pressed the junta to retrieve the back taxes before the statute of limitations is reached.
But it comes at a sensitive time for the military who recently embarked on a much trumpeted "reconciliation" drive.
Junta critics say the revival of the Shin Corp tax issue is vindictive and a fresh effort to block a political comeback from the Shinawatra clan.
They have won every general election since 2001 helped by a fervent following in Thailand's poorer rural north, earning the loathing of the Bangkok-centric elite in the process.
Thaksin's sister Yingluck is also under junta pressure to pay $1 billion in compensation for a rice subsidy program launched by her government that they claim was soaked in graft.
Prayut has pitched himself as a reluctant mediator in Thailand's fraught politics.
But many analysts cast doubt on the army's impartiality given its penchant for coups and support for legal moves against the Shinawatra clan.
