The UNWTO said it had revised its forecast to take into account "the unparalleled introduction of travel restrictions across the world," but added it "should be interpreted with caution in view of the extreme uncertain nature of the current crisis."
The body had predicted at the beginning of the year that international tourism would grow by 3% to 4% in 2020.
"Tourism is among the hardest hit of all economic sectors," the body's secretary general, Zurab Pololikashvili, said in the statement, adding "it is clear" that millions of jobs within the sector are at risk.
The UNWTO noted that small and medium-sized businesses make up around 80% of the tourism sector, which "leads the way" in providing jobs for women, youth, and rural communities.
By comparison, international tourism arrivals declined by 4% in 2009 during the global economic crisis and by just 0.4% in 2003 after the outbreak of Severe Acute Respiratory Syndrome (SARS) which killed 774 people worldwide, it added.
Governments around the world have restricted travel as they scramble to contain the spread of the disease which has claimed over 23,000 lives worldwide since it first emerged in China late last year.
Major cultural events and business conferences have been called off to limit contact between people and airlines have drastically cut back the number of flights they offer. (READ: Beach resort a 'ghost town' as Cyprus tourism hit by virus crisis)
International tourism arrivals rose by 4% in 2019 to 1.5 billion, with France the world's most visited nation, followed by Spain and the United States.
The last time international tourist arrivals posted an annual decline was 2009 when the global economic crisis hit the travel sector hard.