Dec 14 (Reuters) - Concerns over the Omicron coronavirus variant and fresh travel restrictions have led to a spike in hotel booking cancellations globally, online hotel search firm Trivago said on Tuesday, threatening to upend a fragile recovery in tourism. Cancellation rates increased to 35% since November and holiday travel planning was down 10%, the company said, adding that most travelers were choosing domestic destinations. The company did not say what the cancellation rates were prior to November. The Omicron outbreak, first reported in southern Africa, has led to a flurry of new testing rules and border closings, raising concerns ahead of the important Christmas travel season. Multiple events ranging from sports and trade shows to corporate parties have been canceled, also leading to a slowdown in hotel bookings. read more Trivago said overall activity around holiday travel planning has slowed, growing by just 4% since the variant was discovered. By comparison, Christmas travel increased by 34.7% in the same one-to-two-week period in 2019. Last month, the company said it saw an increase of 35% in search traffic in the week leading up to Thanksgiving. U.S. travelers are sticking with Las Vegas and New York as top holiday destinations, the company said. "Much like 2020, domestic travel is the preferred option as consumers look to reunite with friends and family," Trivago said. United Airlines (UAL.O) said on Tuesday it expected around 8 million people to fly during the year-end holiday season, which the company added was higher than the number of people flying during the Thanksgiving period. The airline had previously warned that the Omicron variant was a threat to translantic travel. read more International flight searches from the United States were down between 35% and 39% in the first week of December, according to rival Booking Holdings Inc"s (BKNG.O)travel website Kayak. They were down about 25% just before Thanksgiving, a day before the World Health Organization named the new COVID-19 variant and said it may spread more quickly than other forms, according to Kayak. Fears over the variant have grown after British Prime Minister Boris Johnson warned of a "tidal wave" of new cases, with the country reporting the first publicly confirmed death in the world due to Omicron on Monday.