Workers at Thai Airways International (THAI) are demanding a pay rise for staff, who the national carrier's union claims are being overworked and unfairly taxed.
Union president Damrong Waikanee said yesterday discrepancies have arisen among employees in terms of how much income tax they pay. He said the company may be at fault.
Pilots, mechanics and other staff have expressed their displeasure at the imbalance, Mr Damrong said.
This could cause some to quit if steps are not taken, he warned.
The employees are split into two groups: The first comprises 20,000 people who joined the company in or before 2005; the second batch of 4,000 landed jobs there from 2006.
Mr Damrong said the first group pays less tax than their colleagues who joined later even when their salaries are identical.
He said the company files income tax reports for the first group but the second group must file their own.
For example, a worker in Group A who earns 520,000 baht a month would be taxed 70,000 baht, whereas an employee in Group B would pay twice as much tax.
Mr Damrong said the union has petitioned management to pay staff in Group B higher salaries to make up the difference.
The union has been trying to resolve the issue for seven years but no compromise has been reached, he added.
THAI has paid consultancies over 100 million baht to study ways of streamlining its tax-filing and other systems, the union representative said.
However none of the recommendations were raised at board meetings or acted on, he added.
The company has tried to avoid the issue because it does not want to have file tax returns for its entire workforce, he said.
Meanwhile, the union has urged the National Anti-Corruption Commission (NACC) and the Office of the Auditor General (OAG) to investigate.
The airline has been hit by a wave of challenges in recent weeks.
Later last month, Deputy Transport Minister Pailin Chuchottaworn urged it to procure smaller aircraft to decrease operational and maintenance costs for some of its routes, in a bid to decrease its huge debt and keep up with emerging low-cost airlines that are eating into its profit.
The deputy minister commented on the airline's financial state after a policy-giving visit to THAI's headquarters, part of a bid to decrease debt at seven heavily indebted state enterprises.