Decorated swimmer Donna De Varona, who won two golds at the 1964 Summer Olympics, joined us on "TMZ Live" Thursday, and we asked her about Lia's failed bid to overturn rules preventing transgender women from competing against biological women at the Olympics.

Donna says Lia was never going to get the ban overturned because she didn't get a U.S. Aquatics membership card, which DDV says prevented Lia's challenge from having any standing from the get-go.

Lia -- the first transgender athlete to win an NCAA Division I championship -- wanted to overturn a new gender policy from swimming's governing body, which excludes trans women who fully underwent male puberty from competing against biological women. Lia did not transition until college, so she was excluded under the new policy.

Donna says World Aquatics is trying to accommodate swimmers who don't identify with their birth sex -- Lia included -- but adds not a single swimmer signed up to compete in a new category specifically for transgender swimmers ... which is interesting.

Instead, Donna says trans athletes like Lia only wants to compete against biological women ... which she straight-up says isn't fair.

Donna's been a leader in women's athletics for decades, and she says she's been tracking the number of podium spots across all sports where transgender athletes are taking spots from biological women. The number is astounding ... Donna says there are at least 1,000 instances.