Defending Paralympic champion Andy Lewis says he is "devastated" he will not get a chance to defend his title in Tokyo, but changes to Para-triathlon events mean other British hopefuls will be able to compete for the first time.
Lewis won the PT2 category in Rio, but it is has not been included for 2020.
However, world champion Dave Ellis and Commonwealth winner Jade Jones have both seen their classes added.
"I am devastated by the news my class will not be included," said Lewis.
"The news came out of the blue, and I need more time to digest it and consider my options, but my immediate thoughts remain on defending my ITU world title on the Gold Coast next month and continuing to do what I love - working with young people and schools.
"The Paralympic Games motivates me more than anything else and my love of triathlon doesn't change. So although I am bitterly disappointed, I am still committed to my own training, and to helping young people fulfil their potential and nurture the next generation of triathletes."
Overall, one extra event has been added per gender.
That means 20 additional competitors across the eight categories compared to Para-triathlon's debut at Rio 2016, where 60 athletes contested six medal events.
The women's medal events will be PTWC, PTS2, PTS5, and PTVI, while the men will compete in PTWC, PTS4, PTS5, and PTVI.
Chief executive of British Triathlon Andy Salmon says the decision "comes earlier than expected" and has "disrupted plans to prepare and support our athletes".
"Whilst it gives us more time to prepare, it obviously has some major implications for the team as a whole in terms of athletes missing out on the opportunity to compete on the world's biggest stage," he added.
"It's definitely positive to have four male and four female categories instead of three and three. This means that female wheelchair athletes and male visually impaired athletes are included for the first time.
"However, from a British perspective, we are extremely disappointed that Andy Lewis will not be able to defend his PTS2 Paralympic title, and we have other athletes who will also miss out."
Jones, who will compete for the first time, added: "The news on categories has come earlier than expected but obviously for me and the other female wheelchair athletes it's fantastic.
"Our category wasn't included in Rio, but now we have an opportunity to race on the biggest stage of all in Tokyo."