Tom Daley is "furious" over world swimming bosses' decision to ban transgender athletes from elite women's competitions.
Daley, 28, told reporters at the British LGBT Awards on Friday that the decision by FINA - the governing body for international water sports - is "not on".
After collecting his sports personality of the year award, he told iNews: "I was furious.
"You know, like most queer people, anyone that's told they can't compete or can't do something they love just because of who they are, it's not on.
"It's something I feel really strongly about. Giving trans people the chance to share their side."
According to new FINA rules, transgender women who have gone through male puberty are not allowed to compete in women's events. Their testosterone levels in serum or plasma must have been consistently under 2.5 nmol/L since the age of 12.
By contrast, the regulation state: "Female-to-male transgender athletes (transgender men) are fully eligible to compete in men's swimming competitions."
New 'open' category will open up sport to all
Separately, FINA says it has commissioned a working group to create an "open" category.
It claims it will give "everyone the opportunity to compete at elite level", opening up competitions to those who have "complete androgen insensitivity and therefore could not experience male puberty".
Former Olympic medal-winning swimmer Sharron Davies said FINA is "standing up for fair sport for females", but there has been a bitter backlash elsewhere.
Transgender rights in sport have been put back in the spotlight after the International Rugby League banned transgender women, claiming it would "balance the individual's right to participate".
The surrounding conversation intensified after University of Pennsylvania swimmer Lia Thomas became the first transgender NCAA champion in Division I history after winning the women's 500-yard freestyle earlier this year.
Before that, New Zealand weightlifter Laurel Hubbard last year became the first transgender athlete to compete at the Olympic Games in Tokyo.
President of World Athletics Lord Coe suggested his sport could soon follow swimming, warning that "fairness is non-negotiable".