World Athletics criticised over decision to exclude trans women from female events
World Athletics has decided to exclude transgender women who have gone through male puberty from female events.
The body's president, Lord Coe, said "decisive action" had been taken to "protect the female category in our sport".
Lord Coe told a news conference in Monaco: "The council has agreed to exclude male-to-female transgender athletes, who have been through male puberty, from female world ranking competitions from 31 March.
"In January, World Athletics said it was consulting with member federations on a proposal to change the rules on participation of transgender women in female competitions.
"The issue is thorny as sports try to strike a balance between inclusivity and making sure there is no unfair advantage."
Speaking to Sky News' Mark Austin, Lord Coe said the decision was made in the "best interest of our sport".
"It was a decision based on a very important and fundamental principle and that is, as an International Federation that has a vibrant female element in our sport, it was absolutely vital that we protected the female category."
Transgender athletes competing at elite level would "compromise the integrity of female competition", he added.
When asked how he would respond if a transgender athlete said the decision was unfair, Lord Coe told Sky News: "We don't have any transgender athletes in international competition, that day may come.
"But until we understand a great deal more about the science around this, which we don't, we have to fall back on a fundamental principle and that is that we are here charged with the responsibility of defending the female category, and that is what we have done."
The decision was welcomed by Olympic swimmer Sharron Davies, who tweeted that the rules around the female category should be extended to cover those competing at school level too.
But LGBT campaign group Stonewall described the decision as "disappointing" - and criticised World Athletics for "closing the door" on trans people competing at an international level.
Lord Coe said the decision was open to being challenged and was not "written in stone" - but vowed World Athletics would to "vigorously defend" its position.
He earlier told the news conference a "working group" taskforce will be established in order to do further research on transgender eligibility guidelines, including an independent chair and transgender athletes.
Its remit will be to "consult specifically with transgender athletes to seek views on competing in athletics" - in the hope of reviewing and potentially commissioning additional research.
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The decision to exclude transgender women was made after a consultation with a number of stakeholders including 40 member federations, athletes and coaches, together with community groups including trans organisations, UN experts, the International Olympic Committee (IOC) and Para Athletics, Lord Coe said.
"The majority of those consulted stated that transgender athletes should not be competing in the female category," he said.
"Many believe there is insufficient evidence that trans women do not retain advantage over biological women, and want more evidence that any physical advantages have been ameliorated before they are willing to consider an option for inclusion into the female category."
Differences in sex development athletes
The new rules will also impact athletes with differences in sex development (DSD) - a group of rare conditions involving genes, hormones and reproductive organs, which means a person's sex development is different to others.
Some young people and adults prefer to use the term "intersex", according to the NHS website.
Athletes with DSD include two-times Olympic 800m champion Caster Semenya; 2020 200m Olympic silver medallist Christine Mboma; and Francine Niyonsaba, who finished runner-up to Semenya in the 800m race at the 2016 Olympics.
The maximum amount of plasma testosterone for DSD athletes will be cut in half, Lord Coe told the conference.
The announcement comes after British Athletics called for transgender women to compete with men in an "open category" in February.
The "open" category would replace the current male category, the governing body says - as it demands a law change to preserve women's events for those who were female at birth.
Swimming's world governing body, FINA, voted to effectively ban transgender athletes from competing in women's elite races and explore setting up an "open" category in June last year.