Tobacco giant Philip Morris on Monday pleaded innocent to dodging hundreds of millions of dollars in import taxes, a crime carrying a fine of up to 80 billion baht.
Prosecutors accuse the local unit of the company, which owns the Marlboro and L&M brands, of evading some 20 billion baht in taxes by under-declaring import prices for cigarettes from the Philippines between 2003 and 2006.
In fact, the duty-free end price of the cigarettes was much higher, according to prosecutors.
The company and seven employees pleaded innocent according to a written statement read out by a judge at a pre-trial hearing at a Bangkok court on Monday.
If convicted prosecutors say the company could be fined up to four times of the sum of unpaid tax, while the employees face a maximum of 10 years in jail.
Four foreign executives have also been charged but have left the country in a case that dragged on for a decade.
The company "vigorously" denies the "baseless" allegations, Alejandro Paschalides, managing director of Philip Morris Thailand, said after the hearing.
"We would like to encourage the Thai government to reconsider these meritless charges which will harm Thailand's standing in the trade community and ultimately cause damage to the Thai economy and thus the Thai people," he added in a statement.
The cigarette manufacturer insists that its import valuations complied with World Trade Organization agreements and had been cleared by local customs officials.
The legal issue has simmered since 2006 under the administration of then-prime minister Thaksin Shinawatra, shortly before his ousting in a military coup.
In 2011, the attorney general at the time recommended against charging the tobacco giant, but the prosecution was restarted two years later.
The next hearing will be in October but the trial is likely to drag out for a number of years.