South Korea's tourism body fears a huge drop in visitors, as a diplomatic row with China starts to bite.
The Korea Tourism Organisation (KTO) predicted there could be 4.7 million fewer foreign tourists this year than in 2016 - a drop of about 27%.
China has banned travel agencies from selling package tours to Korea in protest at Seoul allowing a US missile defence system.
Visitors from China made up 46.8% of tourists in South Korea last year.
Tourist numbers into South Korea saw double digit growth in the first two months of the year, the KTO said, but have declined every month since then.
May was particularly bad, with arrivals falling 34.5% against a year ago, and dropping below a million for the first time since the MERS epidemic in mid-2015.
"With the current trend, South Korea's tourism industry could enter a long-term depression," an unnamed KTO official told Yonhap.
The group tour ban came into force after the US military started to deploy the Terminal High Altitude Area Defense (THAAD) system in South Korea.
While supposed to guard against North Korea's shorter-range missiles, it has drawn criticism from China, which claims the powerful radar can probe deep into its territory.
Although the US insists it's a purely defensive weapon, China says it undermines regional security.