Public trust and confidence in public prosecutors and police have been shaken after all charges against Red Bull scion Vorayuth "Boss" Yoovidhya in his fatal hit-and-run case were dropped.

The move has sparked public fury over the impunity enjoyed by the rich in Thailand, with angry netizens pointing out that prisons are used to hold the poor, while the rich never see the inside of a jail cell.

The reasons behind the decision are still unknown to the public and the Office of the Attorney General (OAG) has not given any explanation.

The news immediately sparked public anger. On Twitter, #BossYoovidhya was the top trending hashtag in Thailand in over 200,000 tweets as of 8pm on Friday.

No one from the OAG could be reached for comment on Friday.

Pol Col Kissana Phathanacharoen, deputy spokesman of the Royal Thai Police, told a press conference on Friday that the Department of Southern Bangkok Criminal Litigation decided late last month not to press the remaining charge of reckless driving causing death against Mr Vorayuth, and police agreed with the prosecutors.

The next steps will be to formally withdraw local warrants for the arrest of Mr Vorayuth through the court and ask Interpol to lift its red notice, Pol Col Kissana said.

"The revocation of warrants is a normal procedure in such cases," Pol Col Kissana said while refusing to elaborate on the reasons for the prosecutors' decision.

"Mr Vorayuth could now return to Thailand without any problem. At this moment, we don't know where he [Boss] is," he noted.

Pol Maj Chavalit Laoha-udom, a list MP of the Move Forward Party (MFP) who was a forensic officer gathering and examining evidence in the case at the time said he was upset by prosecutors' decision.

He said he was confident that there was solid evidence against Mr Vorayuth.

Jirawat Aranyakanont, a Bangkok MP for the MFP, called on police and prosecutors to provide reasons for dropping the charge.

"This is unlikely to happen to ordinary people. I wonder if prisons are for locking away poor people only," Mr Jirawat said.

Activist Srisuwan Janya said that the decisions by the prosecutors and police in the case might constitute an act of preferential treatment and he will ask the National Anti-Corruption Commission to investigate next week.

Mr Vorayuth, now 35, was accused of driving his black Ferrari when it hit the rear of a policeman's motorcycle at high speed, dragging his body along Sukhumvit Road before speeding away in the early morning of Sept 3, 2012. The victim was Pol Sgt Maj Wichian Klanprasert, 47, who was based at Thong Lor police station. Mr Vorayuth was then 27 years old.

He postponed hearing charges seven times. It was not until April 27, 2017, that prosecutors finally charged him with reckless driving causing death and failing to help a crash victim.

He fled on a private plane two days before he was due to face the charges.

The speeding charge and reckless driving causing damage to assets were later dropped when the one-year statute of limitations expired. The next charge -- failing to stop and help a crash victim -- expired on Sept 3, 2017.

The last and most serious charge, reckless driving causing death, would have remained until 2027, but has now been dropped.