The case against the Thai heir to the Red Bull billions accused of a fatal hit-and-run was "compromised" from beginning to end, the premier's office said Tuesday, calling the entire procedure "dishonest".
The announcement comes a month after charges against Vorayuth "Boss" Yoovidhya were dropped by the police and attorney general's office, prompting outrage from ordinary Thais who saw it as another example of the impunity enjoyed by the kingdom's elite.
The controversy surrounding the case comes during a tense period for the government as a burgeoning pro-democracy movement has gathered pace with near-daily protests across the country.
Vorayuth is accused of killing a police officer in a hit-and-run in 2012 after crashing his Ferrari in Bangkok, and for using cocaine at the time of the accident.
He fled the country on his private jet in 2017, but his return was widely expected after authorities revealed in July that all charges against him had been dropped.
But a huge public outcry -- including hundreds of thousands of people using the hashtag #BoycottRedBull on social media -- prompted three new probes.
One, set up by Prime Minister Prayut Chan-O-Cha's office -- concluded Tuesday that the entire investigation had been "illegal".
"The case's investigation was compromised," committee chairman Vicha Mahakun told reporters. "It was done dishonestly and with conspiracy to weaken the case."
Prayut, whose administration has long enjoyed close alliances with the kingdom's billionaire clans, said the Red Bull case cast doubt on the country's legal and criminal systems.
"There were policemen, attorney generals and people in politics involved," he said, promising "legal and ethics actions" would be taken against them.
The report from the premier's office comes a week after Bangkok's Criminal Court issued a fresh arrest warrant for Vorayuth.
As the grandson to Red Bull co-founder Chaleo Yoovidhya, the 38-year-old is part of a clan which boasts a net worth of $20.2 billion -- making it Thailand's second richest family, according to Forbes.
Several other incidents added to public suspicions of a cover-up, including the death in a motorcycle accident last month of a man police presented years after the fact as a witness who had cleared Vorayuth of involvement.
On Tuesday a small group of demonstrators rallied in central Bangkok, calling again for the former army chief Prayut's removal from office.
Other protesters have recently waded into taboo territory by calling for reforms to the country's unassailable monarchy.
One of them, student leader Jutatip Sirikhan, was arrested Tuesday -- the latest among dozens who face multiple charges including sedition.