Transgender women should not play elite women's rugby, World Rugby said on Friday following a wide-ranging review of their policies relating to transgender players.
New guidelines have been drawn up to cover the participation of transgender athletes in men and women's contact rugby where it is possible to do so safely and fairly.
As a result, transgender men are permitted to participate in men's contact rugby but the same does not apply to transgender women in women's rugby.
"Given the best available evidence for the effects of testosterone reduction on these physical attributes for transgender women, it was concluded that safety and fairness cannot presently be assured for women competing against transwomen in contact rugby," said a World Rugby statement.
"As a result, the new guidelines do not recommend that transwomen play women's contact rugby on safety grounds at the elite and international level of the game where size, strength, power and speed are crucial for both risk and performance, but do not preclude national unions from flexibility in their application of the guidelines at the community level of the game.
"Transmen are permitted to participate in men's contact rugby."
The issue of trans women competing in women's sport has become a thorny debate.
Several high-profile female athletes including Martina Navratilova, Kelly Holmes and Paula Radcliffe have spoken out about the possible damage to women's sport of allowing potentially more powerful trans women to compete.