Riley Gaines with Lia Thomas (left), 22, who became the first transgender swimmer to win America's top university swimming championship in the female category last March.
The sport's governing body has introduced an 'open' category for those born male as well as trans or non-binary competitors, while a 'female' category will be restricted to those with a birth sex of female.
The policy comes into effect on September 1 and will apply to competitions under Swim England's auspices including those organised by member regions and clubs.
Swim England said hormonal therapy reduced performance advantage but was 'insufficient to negate it completely and trans females therefore likely retain an advantage over their cisgender peers'.
'As a result, Swim England believes the restriction of certain competition to birth sex females to be justified and proportionate in the pursuit of fair competition,' it added.
The policy contains provision for self-identification at lower-level events such as recreational races.
Last year swimming's world governing body FINA voted to ban those who have gone through male puberty from women's events. It comes amid controversy in the US after Lia Thomas, 22, became the first transgender swimmer to win America's top university swimming championship in the female category last March.
Protesters argued that as she was born a man she had an unfair advantage over her fellow competitors. She began her hormone replacement therapy in May 2019 and adopted her new name on New Year's Day in 2020.
It comes amid controversy in the US after Lia Thomas became the first transgender athlete to win America's top university sports competition for her performance in a 500 yard freestyle swim race last year.
I don't envy to woke folks, that mixed signals must drive they already mushy little brains crazy ...