The performance in international English proficiency tests is also inferior to other countries in the region," Mr Treenuch said.
The average Thai test-taker typically scores lower than those in Singapore, Malaysia, the Philippines, Indonesia, Vietnam and Myanmar on both the International English Language Testing System and the Test of English as a Foreign Language.
Thailand ranks 55th out of 60 countries on the English Proficiency Index, the world's major ranking of English-language skills. That is the lowest among Southeast Asian countries.
Thais struggle with English even though it is a mandatory subject from primary school through high school and despite 20% of Thailand's GDP coming from tourism.
"Classrooms in Thailand put too much emphasis on grammar," said Sarah Wilson, an English teacher who has taught in Thai schools for more than 10 years. "They focus on studying for tests rather than being able to converse and use English in real-life situations."
"Many students struggle forming sentences on their own," said Krittapot Jiravat, a private English tutor. "All they want to do is study for exams and tests, because they think they don't have any other purposes beyond that."
"Many professionals have to study after work when they realise that English is important in their career," he said.
"Once the single market is launched, the job market will become more competitive," Mr Treenuch said. "People with higher skills in English will be at advantage."
Workers with stronger English skills typically earn 30%-50% more pay than those with weaker proficiency, according to the Harvard Business Review.