Lia Thomas' participation in NCAA races has caused a USA Swimming official to resign ©Penn State

A USA Swimming official who has been with the organisation for 30 years has resigned in protest of a transgender athlete participating in the sport.

Cynthia Millen was due to officiate on the United States Paralympics Swimming National Championships in Greensboro in North Carolina earlier this month.

When learning of Lia Thomas' participation in the event, representing the University of Pennsylvania, Millen pulled out, sighting her unwillingness to support transgender athletes competing in women's sport.

"I told my fellow officials that I can no longer participate in a sport which allows biological men to compete against women. Everything fair about swimming is being destroyed," she wrote in a letter to USA Swimming as reported by the Washington Times.

Millen added in the letter: "I don’t mean to be critical of Lia — whatever’s going on, Lia’s a child of God, a precious person — but bodies swim against bodies.

"That’s a male body swimming against females, and that male body can never change. 

"That male body will always be a male body."

Millen added to the Times: "If Lia came on my deck as a referee, I would pull the coach aside and say, 'Lia can swim, but Lia can swim exhibition or a time trial. Lia cannot compete against those women because that’s not fair'.

"This is not right because by doing this, we’re supporting this.

"There are no swim meets if there are no officials."

Under National Collegiate Athletic Association (NCAA) rules, Thomas is allowed to compete in women's events as she has completed the necessary one year of testosterone suppression treatment.

Thomas competed for three years on the men's swim team, before switching to the women's this year.

She is now enjoying a record-breaking season and ranks first in the NCAA rankings in the women's 200 metres and 500m freestyle.

The 22-year-old has come close to NCAA record times set by US Olympic gold medallists Missy Franklin (200m) and Katie Ledecky (500m).

Thomas has also come within 1min of Ledecky's 1,650m NCAA record time and has dominated competition in the event this year.

The Daily Mail reported some parents vented their frustration in a letter to the NCAA and the University of Pennsylvania.

Two female swimmers at the university anonymously told Outkick Thomas' participation was unfair and demoralising.

USA Swimming chief executive Tim Hinchey told the Inside with Brett Hawke Podcast Thomas was not a member of the organisation.