The forecast from the Madrid-based World Tourism Organization comes as Europe is grappling with a surge in infections and as a new heavily mutated Covid-19 variant, dubbed Omicron, spreads across the globe.
International tourist arrivals will this year remain 70-75 percent below the 1.5 billion arrivals recorded in 2019 before the pandemic hit, a similar decline as in 2020, according to the body.
The global tourism sector already lost $2.0 trillion (1.78 trillion euros) in revenues last year due to the pandemic, according to the UNWTO, making it one of sectors hit hardest by the health crisis.
While the UN body charged with promoting tourism does not have an estimate for how the sector will perform next year, its medium-term outlook is not encouraging.
"Despite the recent improvements, uneven vaccination rates around the world and new Covid-19 strains" such as the Delta variant and Omicron "could impact the already slow and fragile recovery," it said in a statement.
The introduction of fresh virus restrictions and lockdowns in several nations in recent weeks shows how "it's a very unpredictable situation," UNWTO head Zurab Pololikashvili told AFP.
"It's a historical crisis in the tourism industry but again tourism has the power to recover quite fast," he added ahead of the start of the WTO's annual general assembly in Madrid on Tuesday.
"I really hope that 2022 will be much better than 2021."
While international tourism has taken a hit from the outbreak of disease in the past, the coronavirus is unprecedented in its geographical spread.
In addition to virus-related travel restrictions, the sector is also grappling with the economic strain caused by the pandemic, the spike in oils prices and the disruption of supply chains, the UNWTO said.
Pololikashvili urged nations to harmonise their virus protocols and restrictions because tourists "are confused and they don't know how to travel".
International tourist arrivals "rebounded" during the summer season in the Northern Hemisphere thanks to increased travel confidence, rapid vaccination and the easing of entry restrictions in many nations, the UNWTO said.
"Despite the improvement in the third quarter, the pace of recovery remains uneven across world regions due to varying degrees of mobility restrictions, vaccination rates and traveller confidence," it added.
Arrivals in some islands in the Caribbean and South Asia, and well as some destinations in southern Europe, came close to, or sometimes exceeded pre-pandemic levels in the third quarter.
Other countries however hardly saw any tourists at all, particularly in Asia and the Pacific, where arrivals were down 95 percent compared to 2019 as many destinations remained closed to non-essential travel.
A total of 46 destinations -- 21 percent of all destinations worldwide -- currently have their borders completely closed to tourists, according to the UNWTO.
A further 55 have their borders partially closed to foreign visitors, while just four nations have lifted all virus-related restrictions -- Colombia, Costa Rica, Dominican Republic and Mexico.
The future of the travel sector will be in focus at the WTO annual general assembly, which will run until Friday.
The event -- which brings together representatives from 159 members states of the UN body -- was original scheduled to be held in Marrakesh.
But Morocco in late October decided not to host the event due to the rise in Covid-19 cases in many countries.
Before the pandemic, the tourism sector accounted for about 10 percent of the world's gross domestic product and jobs.