Out tennis legend Martina Navratilova has penned an op-ed against transgender athletes competing in women's sports.

Navratilova's column, titled “The Rules on Trans Athletes Reward Cheats and Punish the Innocent,” appeared in the Sunday Times of London.

In December, Navratilova, who came out lesbian in 1981, was criticized for tweeting: “You can't just proclaim yourself a female and be able to compete against women. There must be some standards, and having a penis and competing as a woman would not fit that standard.”

The backlash forced Navratilova to delete the tweet and promise to “educate” herself on the subject.

In her op-ed, Navratilova said that her “research” on the subject had strengthened her views.

“To put the argument at its most basic: a man can decide to be female, take hormones if required by whatever sporting organization is concerned, win everything in sight and perhaps earn a small fortune, and then reverse his decision and go back to making babies if he so desires,” Navratilova wrote.

“It’s insane and it’s cheating. I am happy to address a transgender woman in whatever form she prefers, but I would not be happy to compete against her. It would not be fair.”

“Simply reducing hormone levels – the prescription most sports have adopted – does not solve the problem. A man builds up muscle and bone density, as well as a greater number of oxygen-carrying red blood cells, from childhood,” Navratilova said, a reference to guidelines that specify testosterone levels for transgender women to compete.

Rachel McKinnon, who in October became the first transgender woman to win a world title in track cycling, responded in a tweet: “Remember when I said that it looked like Martina Navratilova is transphobic? Holy. Fuck. She went FULL transphobe.”