After weeks of insisting that the Tokyo Games would take place on schedule this summer, Olympic officials revealed Sunday they are considering a postponement, which could mark the first step in ultimately delaying the world’s largest sporting event because of the novel coronavirus.

Facing mounting pressure and growing criticism, Thomas Bach, president of the International Olympic Committee, wrote a letter to Olympic athletes Sunday in which he said canceling the Summer Games altogether is not an option and vowed that a final decision on postponement would be made within the next four weeks.

The message prompted a strong backlash Sunday night when the Canadian Olympic Committee said it would not send its athletes to compete in Tokyo this summer, the strongest statement to date against the IOC’s preference to stick to its schedule.

“This is not solely about athlete health — it is about public health,” the committee said in a statement, in which it called for a postponement. “With COVID-19 and the associated risks, it is not safe for our athletes, and the health and safety of their families and the broader Canadian community for athletes to continue training towards these Games. In fact, it runs counter to the public health advice which we urge all Canadians to follow.”

Canada is the first country to refuse participation in these Summer Olympics, dealing the IOC a devastating blow hours after the international governing body sought to assuage its growing detractors and critics with its message to athletes.

“Together with all the stakeholders, we have started detailed discussions today to complete our assessment of the rapid development of the worldwide health situation and its impact on the Olympic Games, including a scenario of postponement,” Bach wrote.

Hours after Bach’s letter but before the Canadian delegation’s announcement, Japanese Prime Minister Shinzo Abe told a parliamentary committee Monday morning that postponing the Tokyo Games may become a consideration if the event cannot be staged in a “complete manner,” marking the first time Abe has publicly entertained the idea of a delay.

“If that is difficult, we would have no choice but to decide to postpone with athletes in the first priority,” Abe said, adding that canceling the Tokyo Games altogether is not an option.

Later on Monday, Yoshiro Mori, president of the Tokyo 2020 Organizing Committee, said the IOC and Japan would like to “closely examine” the various scenarios open to them over the next four weeks, although he hinted that organizers would prefer to keep the Olympics within 2020.

“What we are going to do before anything else is to start by simulating about whether we postpone one month, three months, five months, one year,” Mori said. “We need to make a simulation about the various scenarios.”

The public concessions come as the IOC has found itself increasingly under fire, facing heavy criticism from many athletes and sports federations. USA Swimming and USA Track and Field are among the organizations that have publicly called for a one-year postponement in recent days, and athletes from around the world have voiced their concerns about competing in Tokyo this summer. Many have struggled to train and prepare for the Olympics while also abiding by safety warnings from public health officials and government-backed restrictions.

Bach said in his letter that deciding on the fate of the Tokyo Olympics now “would still be premature.”

“This uncertainty rocks our nerves and raises or strengthens doubts about a positive future; it destroys hope,” he wrote. “Some even have to fear for their very existence. This uncertainty stems from the fact that, at this moment, nobody can really make fully reliable statements about the duration of this fight against the virus. This is true for sport, science, the media, politics, and all of society. Therefore also the IOC can unfortunately not answer all your questions.”

The letter didn’t stop the calls for an Olympic postponement. Sebastian Coe, head of World Athletics, the international governing body for track and field, wrote a letter of his own to Bach on Sunday and said keeping with a July-August schedule is “neither feasible nor desirable.”

“No one wants to see the Olympic Games postponed but as I have said publicly, we cannot hold the event at all cost, certainly not at the cost of athlete safety, and a decision on the Olympic Games may become very obvious very quickly,” Coe wrote in the letter. “I believe that time has come and we owe it to our athletes to give them respite where we can. And in this matter, I believe we can.”

Late Sunday night, Australia added itself to the chorus. While not as strident as Canada, Australia advised athletes to prepare for an Olympics in 2021 and said a team “could not be assembled” for 2020 given the global circumstances. “It’s clear the Games can’t be held in July,” Ian Chesterman, the Australian chef de mission for Tokyo, said in a statement.

Sarah Hirshland, chief executive of the U.S. Olympic and Paralympic Committee, issued a joint statement Sunday with Han Xiao, chair of the organization’s athletes advisory council, saying the IOC letter was “an important step in providing clarity, but our athlete community continues to face enormous ambiguity surrounding the 2020 Games in Tokyo.”

The USOPC officials said they’re “eager to continue to explore alternatives to ensure all athletes have a robust and fulfilling Olympic and Paralympic experience, regardless of when that can safely occur. Together we will find solutions that keep the spirit of the Games alive.”

The U.S. Anti-Doping Agency also called for a postponement Sunday. Travis Tygart, USADA’s chief executive, told The Washington Post: “We agree the Games should be postponed, unfortunately, up to a year in fairness to athletes whose lives have been upended and to ensure they don’t potentially become the dirtiest Games ever due to the significant reduction of anti-doping efforts due to covid-19.”

By acknowledging postponement as a possibility, the IOC has relaxed its tone while still holding on to hope that the Tokyo Games can go on as scheduled.

“It is our experience as athletes that you must always be ready to adapt to new situations,” Bach wrote. “For this reason we have, as indicated before, been thinking in different scenarios and are adapting them almost day by day.”

Bach did not outline possible scenarios. A one-year delay appears to be the most likely option, though organizers also could consider a postponement of a few months or perhaps up to two years. While an Olympics has never been postponed, several have taken place later on the calendar, including the 2000 Sydney and 1988 Seoul Games, which both took place in late September, and the 1964 Tokyo and the 1968 Mexico City Games, which took place in October.

As concerns over the rapid spread of the disease have grown in recent weeks, pressure has mounted on Olympic officials to make a difficult decision on an event that poses a massive problem for organizers and athletes alike. As they publicly encouraged athletes to continue training for this summer, Olympic officials privately discussed holding the Summer Games without spectators or rescheduling the event for a later date.

The Olympics, scheduled to begin July 24, draw together more than 11,000 athletes and 25,000 journalists from more than 200 countries — plus hundreds of thousands of foreign tourists — running counter to the practice of social distancing advised by public health officials. The Paralympics, scheduled to begin Aug. 25, were expected to attract 4,400 participants from around the world.

The coronavirus has disrupted athletic schedules and canceled key qualifying events all over the globe. As gyms and facilities closed down — including two U.S. Olympic training centers — many athletes struggled to find a place to train. Still, at an IOC executive board meeting March 3, a spokesman said the organization had not even discussed contingency plans and vowed the Games would begin on time.

Bach spoke with stakeholders last week, sharing the same message, and the IOC issued a communique saying, “The IOC encourages all athletes to continue to prepare for the Olympic Games Tokyo 2020 as best they can.”

Postponing an Olympics is no easy task; it would involve numerous stakeholders, sponsors, countries, sports federations and national organizing committees. Experts say any sort of delay probably will pose logistical problems in terms of the global sporting calendar and would upend the schedules for athletes, who have been targeting the summer of 2020 and built their lives and training routines around peaking competitively this year.

In his letter to athletes, Bach called postponement an “extremely complex challenge."

“A number of critical venues needed for the Games could potentially not be available anymore,” Bach said. “The situations with millions of nights already booked in hotels is extremely difficult to handle, and the international sports calendar for at least 33 Olympic sports would have to be adapted. These are just a few of many, many more challenges.”

Hayley Wickenheiser, who won four gold medals and a silver in the Winter Olympics with the Canadian women’s ice hockey team, and made an appearance in the 2000 Summer Games on Canada’s softball team, said the Canadian Olympic Committee’s statement Sunday made her “proud” of her country.

Wickenheiser is also a member of the IOC and its Athletes’ Commission, and she had made waves Tuesday by posting comments online in which she accused the organization of being “insensitive and irresponsible” in continuing with the planned dates this summer for the Tokyo Games. Reached by phone Sunday, she said she heard from “senior leadership” at the IOC who “weren’t very happy” with her remarks, but she pointed to her recent experience “on the front lines in emergency rooms” as having dramatically altered her own outlook on the crisis.

“I’ve seen this virus progress,” said Wickenheiser, 41, who noted she was in her final year of medical school. “Quite honestly, I didn’t give the severity of it the full attention that it deserved until a couple of weeks ago. Seeing a few things in the emergency room quickly changed my perspective on this. … Looking forward, the medical evidence just doesn’t lend itself to the world being ready to hold an Olympics in four months.”

Wickenheiser asserted that she thought postponement of the Tokyo Olympics was now “inevitable,” but added that the IOC and the Japanese organizing committee deserve some time to make new arrangements.

“This is a very complex event with a lot of moving targets and a lot of pieces involved, so to just come out with an answer tomorrow isn’t necessarily fair,” she said, “but I think they have no choice but to postpone.”

The IOC has said it will rely on counsel from the World Health Organization and prioritize health and safety, but there’s also big money at stake, and postponement could require heavy changes over major contracts. The Olympics are a costly undertaking, and these Summer Games were expected to carry a price tag of $12.6 billion — though some Japanese estimates have suggested the actual costs would ultimately be much higher.

The Games also generate big revenue. The IOC, a nonprofit organization, brought in more than $5 billion during the most recent four-year Olympic cycle, nearly three-quarters of which came from broadcast rights. NBC, the rights-holder in the United States, contributes about half of that and thus carries a lot of sway with the IOC. Comcast, NBC’s parent company, has said insurance coverage would ensure the network doesn’t suffer losses, though it would miss out on Olympic-related advertising revenue.

“These are extraordinary and unprecedented times, and we fully support the IOC’s decision to step up its scenario-planning for the Tokyo Olympics,” an NBC spokesman said in a statement Sunday. “We are prepared to stand behind any decision made by the IOC, the Japanese government, and the world health officials with whom they are working regarding the Tokyo Olympics.”

Simon Denyer in Tokyo and Adam Kilgore in Washington contributed to this report.