The International Olympic Committee must act decisively by postponing the Tokyo 2020 Games because of the impact of the coronavirus pandemic, a British athlete has told the Guardian.

Guy Learmonth, who captained the GB team at the European Indoor Championships last year and is hoping to be picked in the 800m, has become the first British athlete to warn that the risks of going to Tokyo in July are not worth it in the current climate.

“We have no idea how bad this is going to get, and what we have seen so far might be the tip of the iceberg,” Learmonth said. “Of course the IOC and the whole world wants a successful Olympics. But for that to happen I strongly believe the event needs to be postponed – unless the authorities can guarantee it will be business as usual, which I don’t believe they can.

“Purely from an athlete’s point of view here, we need some clarity, transparency and flexibility. Because the press conference that I saw from the Japanese prime minister didn’t exactly fill me full of confidence that these Games are going to go ahead whatsoever.”

Learmonth’s comments come as the Diamond League is set to be the latest sporting event to be affected by Covid‑19. An athlete who was due to compete in one of the two Diamond League meetings in China in May has told the Guardian that both are “100% off” – while the season opener in Doha in April is also expected to be postponed or cancelled in the coming days.

“We have to be realistic,” said Learmonth, who reached the world championships semi-finals in 2017 and believes that, at 27, he is in the best shape of his life. “If the British government are saying the peak is going to hit us around May or June there might not even be any Olympic trials. The Australians have already had theirs cancelled and I fear that this is going to be happening a lot throughout Europe as the weeks and months go by.

“I’d be happy if they postponed until at least October – or maybe later to 2021 or 2022. At least that would give the athletes time to now plan, train, and more importantly, time for this virus to settle down.”

Martyn Rooney, who has spearheaded Britain to one Olympic and three world 4x400m relay medals, also urged the IOC and World Athletics to provide greater clarity – especially if Tokyo does go ahead.

Guy Learmonth says athletes would appreciate a delayed Games. Photograph: Ian MacNicol/Getty Images

“I think most athletes are pragmatic and aware that the health and well‑being of not only the athletes but the teams around them are most important,” he said. “If a mass participation event is a high risk then the Olympics is not going to happen and as athletes there’s nothing we can do about it.

“The worry is it goes ahead and athletes haven’t got the chance to qualify. I’d be pissed off if I knew I’m in great shape and I didn’t get the chance to race in good meets and get the necessary time and points to qualify.”

Rooney’s comments came after it was announced that the Olympic flame handover in Athens next week would be held behind closed doors because of coronavirus concerns. But the Japan prime minister, Shinzo Abe, insisted over the weekend that the Games will go ahead as planned in July.

Dai Greene, the 2011 400m hurdles world champion, has told the Guardian he agreed with Rooney – and that the IOC and World Athletics must show more flexibility to allow athletes to qualify if required.

“Currently there are a lot of unknowns for athletes,” said Greene, who has had to cancel his training camp in Florida and will not be able to compete during the US season. “If we cannot compete internationally or domestically during the next few months then a lot of athletes will not be able to qualify for Tokyo. That’s regardless of whether the Games will go ahead at all.”

Greene also warned that many athletes were worried about the financial implications of not being able to compete. “Large portions of the athletics community are not funded or are not funded enough to make a living. We don’t have clubs who give us wages, we have nothing to fall back on, it’s a worrying time.”

Meanwhile World Athletics has insisted it would adapt its qualification system if more events were cancelled in the weeks and months ahead. “Athletics is in a relatively good position because we have a very flexible system and the qualification period began more than 10 months ago,” a spokesperson insisted. “We still have two months to go before the international outdoor season begins in earnest and much can change in that time, as it has in the last two months.

“While many international events are being cancelled, some athletes will still be able to do further qualifying competitions in their own countries over the coming months,” they said.

“We will publish a full ‘Road to the Olympic Games’ tracking tool on our website by the end of March to help athletes to see where they stand. If the cancellation of events and restrictions on travel begin to disadvantage certain countries or athletes, we will adjust it, and we will also assess any individual cases that are brought to us.”