The final appeals court for global sports further blurred the line separating male and female athletes on Monday, ruling that a common factor in distinguishing the sexes —  the level of natural testosterone in an athlete's body — is insufficient to bar some women from competing against females.

The Court of Arbitration for Sport’s decision, suspending the practice of "hyperandrogenism regulation" by track and field's governing body, the International Association of Athletics Federations, is the latest demonstration that biological gender is part of a spectrum, not a this-or-that definition easily divided for matters such as sport. It also leaves officials wondering how and where to set the boundaries between male and female competition.

The issue frustrates governing bodies and divides fans and athletes. Among those who testified in support of the IAAF policy was the British marathon runner Paula Radcliffe, who holds the event's world record among women. According to the ruling, Radcliffe said that elevated testosterone levels "make the competition unequal in a way greater than simple natural talent and dedication." She said that other top athletes shared her view.

"The concern remains that their bodies respond in different, stronger ways to training and racing than women with normal testosterone levels, and that this renders the competition fundamentally unfair," Radcliffe said of hyperandrogenic athletes.

The court recognised the complexity of an issue that so many, for so long, had considered a simple matter: Is the athlete a man or woman?

The panel complimented the IAAF for acting with "conspicuous diligence and good faith" in creating and implementing the standards, underlining the difficulty of drawing scientific lines between the sexes. But the panel said it "was unable to conclude that hyperandrogenic female athletes may benefit from such a significant performance advantage that it is necessary to exclude them from competing in the female category."

"Although athletics events are divided into discrete male and female categories, sex in humans is not simply binary," the panel added. "As it was put during the hearing: 'Nature is not neat.' There is no single determinant of sex."

It continued: "Nevertheless, since there are separate categories of male and female competition, it is necessary for the IAAF to formulate a basis for the division of athletes into male and female categories for the benefit of the broad class of female athletes. The basis chosen should be necessary, reasonable and proportionate to the legitimate objective being pursued."

The panel was ruling specifically on track and field's regulation, but the Court of Arbitration for Sport's wide jurisdiction means its decision is likely to serve as a precedent for other sports around the world.

It was just the latest high-profile case to confront the intensely disputed boundaries of fairness in track and field in recent years. The South African sprinter Oscar Pistorius, for example, eventually won the right to compete at the 2012 Summer Olympics in London using prosthetics. In a case more clearly linked to that of Chand, the South African runner Caster Semenya in 2009 was barred from competition and reinstated nearly a year later after a series of gender tests and much publicity.

Two opponents of Semenya's objected to her participation in the 2009 World Championships in Berlin, where she won gold in the 800 metres.

In 2011, the IAAF established guidelines requiring female athletes to have testosterone levels below 10 nanomoles per liter, the lower end of the male range.  At the London Olympics, four female athletes, all 18-21 years old and from rural areas of developing countries, were flagged for high levels of natural testosterone. Each of them subsequently had surgery to remove internal testes, which produce testosterone, as well as procedures not required for resuming competition: feminising vaginoplasty, estrogen replacement therapy and a reduction in the size of the clitoris.  The Chand decision will force the IAAF to devise a new way to divide the spectrum. The appeals court ruled that it remained inappropriate to subject athletes to gender verification, external genitalia examinations or chromosome testing. It declared that there was no dispute that Chand was a woman, and no one suggested that her levels of testosterone were anything but natural.

"I am shocked and heartened by CAS' decision," said Katrina Karkazis of the Center for Biomedical Ethics at Stanford University, who helped argue Chand's case. "I didn't think it was our time. Dutee has made history with her courageous decision to challenge a policy she felt was unfair to her and to all women athletes. It's a victory for women's equality in sport. And I'm thrilled she can just now run."