The TAT now forecasts only 14 million to 16 million foreign visitors this year, a sharp drop from 33.8 million projected in March.
Last year's foreign arrivals were a record 39.8 million.
TAT Governor Yuthasak Supasorn said that tourism is crucial to Thailand.
"Spending from foreign tourists amounted to 1.93 trillion baht (59.9 billion U.S. dollars), that accounted for 11 percent of the gross domestic product last year," the governor said.
Yuthasak hinted that foreign visitors will return to Thailand in October, as high season begins.
"We are pushing in for domestic tourism to start in June or July, but that also depends on the outbreak situation and travel restrictions," he added.
Thailand has reported 3,015 cases and 56 deaths since the outbreak began in the country in January.
The TAT governor said that from January to March, foreign tourist numbers in Thailand slumped 38 percent to 6.69 million, with the number of Chinese visitors, Thailand's biggest source of tourists, plunging 60 percent to 1.25 million.
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