Ikee was one of Japan's big hopes for Tokyo 2020 after winning six golds at last year's Asian Games.
Rikako Ikee, one of the potential stars of Tokyo 2020, has been diagnosed with leukaemia.
The 18-year-old returned home to Japan after falling ill earlier this month during a training camp in Australia.
Ikee announced the news on her Twitter account on Tuesday, addressing the statement to "those who have encouraged me and people it may concern".
"Thank you for your constant support. I was feeling unwell and urgently came home from Australia, underwent testing and the diagnosis was leukaemia.
"I still cannot believe this and it's a confusing situation for me. But with treatment, this is a disease which can be cured." - Rikako Ikee
Just two weeks ago, Ikee was named Japan's Swimmer of the Year for 2018 after winning six golds at the Asian Games in Jakarta.
Born in Tokyo, she was set to carry the hopes and dreams of the host nation in the pool.
But Ikee said she had "no choice" but to miss April's national championships which serve as qualifiers for July's World Championships in Gwangju, South Korea.
She added, "I will now take a little rest and focus on treatment so that I can show an even stronger Rikako Ikee as soon as possible."
The Japan Swimming Federation later held an urgent news conference with an update on Ikee's condition.
Ikee marked herself out as one for the future with victory in the 50m and 100m butterfly at the 2015 World Junior Championships.
She competed in no fewer than seven events at Rio 2016 - four individual and three relay - but failed to come away with a medal.
But she did give notice of her talent in the 100 fly, winning her semi-final in a new national record thanks to a blistering second length.
Ikee went even faster in the final, but could only finish fifth behind Sarah Sjostrom who took gold in a new world record.
Ikee later claimed four victories at the Asian Championships before retaining her two butterfly titles at the 2017 World Juniors as well as taking the 50m freestyle crown.
2018 saw her move definitively into swimming's top rank.
In August's Pan Pacific Championships in Tokyo, Ikee won the 100m butterfly and took second in the 200m freestyle, ahead of Katie Ledecky but behind Canada's Taylor Ruck, in a new Asian record.
Then came her history-making performance at the Asian Games.
Ikee led off Japan's 4x100m freestyle relay team with 53.60 seconds, a new Games record, as they edged out China for victory.
She then won the 50 fly, 100 free, 100 fly and 50 free, all in Games records, and the 4x100m medley relay.
Her tally of six golds and two silvers saw her become the first woman to be named Asian Games MVP.
Only North Korean shooter So Gil-san, who claimed seven golds and a silver at New Delhi in 1982, has won more titles at a single Asian Games.
Ikee's rematch with Sjostrom at Tokyo 2020 promised to be one of the highlights of the Games, and she beat the Swede in the 100 fly at November's FINA World Cup Tokyo in a new short course national record.
Ikee's early diagnosis improves her chances of making a full recovery.
According to cancer.org, acute lymphocytic leukaemia is the most common form of the disease in children and teenagers.
It is often attacked with chemotherapy with the nature of the illness determining how long and how intensively it is administered.
Among the alternative treatments are immunotherapy and radiation therapy.