More than 150,000 foreign nationals need to have their tourist visas renewed by Sept 26 or face a charge of overstaying, warned Pol Col Pakpong Sai-ubol, deputy spokesman of the Immigration Bureau.
"Overstaying the tourist visa is punishable by both a jail term and fine under the Immigration Act," he said, adding offenders would also be deported to their countries of origin.
According to the law, anyone who has overstayed their visa for more than 90 days is barred from re-entering the kingdom for one year. The blacklist extends to a lifetime if the overstaying period exceeds 10 years.
Many foreign visitors were stranded in Thailand at the onset of the Covid-19 pandemic in late March when international flights were suspended and borders closed to stave off the transmission of the virus.
The visitors were given a reprieve where they were able to remain in the country even after their visas had expired.
But the grace period ends on Saturday which has also been set as a deadline for having their visas renewed.
Failure to report to obtain a visa renewal by the deadline will incur a daily fine and lead to authorities tracking down tourists at addresses kept in the IB's database.
Pol Col Pakpong said tourists will be arrested and face legal action.
He added more than 150,000 foreign nationals holding tourist visas are currently in Thailand although the deputy spokesman did not give an exact number.
A source in the IB said authorities were concerned some stranded tourists might not turn up to request visa renewals.
They might prefer to remain in the kingdom illegally if the virus has not been tackled in their home countries.
The source added some stranded tourists might also take up employment illegally as a means of gaining financial support and refrain from contacting their respective embassies in Thailand.
They fear they might be repatriated to their Covid 19-ravaged countries.
Meanwhile, foreign business operators in Phuket cried foul over the end of the visa amnesty.
In a phone interview with the Bangkok Post, Volodymyr Fedorovych, the Ukrainian owner of a boat charter company, said his boats were locked down in March and he had to release two local staff in April.
The province issued an order that led to the closure of tourism-related businesses on the island to prevent the spread of Covid-19 on April 11.
Mr Fedorovych said he had been working in Thailand for more than five years. He holds a Non-Immigrant B-Visa and has it renewed every year.
He said it expired in April, but the government had granted an automatic visa extension.
It permits aliens to stay in Thailand temporarily until Sept 26.
However, Mr Fedorovych said the Phuket immigration office on Aug 6 denied him a visa renewal because his business was not employing local staff.
But re-employing staff requires past records of their employment including three months of the employer's contribution to the social security fund.