Football's world governing body, Fifa, has ruled that a leading Mexican woman footballer is not eligible to play for a professional men's club.
Dominguez has played for a professional women's side in the US
Maribel Dominguez signed for Mexico's second-division club Celaya this week - a move which would have been a first in North and Central America's sport.
But Fifa's executive committee said that "there must be a clear separation between men's and women's football".
Dominguez, 26, has scored 42 goals in 43 games for the national women's team.
The decision by Fifa's executive committee was taken after a weekend meeting in Zurich, Switzerland.
In a written statement, it stressed that the gender-separation principle in football should be maintained.
"This is laid down in league football and in international matches by the existence of gender-specific competitions, and the Laws of the Game and Fifa's regulations do not provide for any exception," the statement said.
Celaya agreed to abide by the Fifa ruling. It also said that signing the player was a publicity stunt.
Dominguez was Mexico's top scorer at the Athens Olympic Games, and was also ranked 25th in Fifa's top women players in the world for 2004.
The striker - who is nicknamed "Marigoal" - had said earlier this week that she would begin a special training regime to get match fit by mid-January.
"The hard thing is going to be equalling the physical force of the men, but the technique, the desire, the willpower, those are things I already have," she said.
Last year Italian team Perugia announced it had made an offer to German World Cup winner Birgit Prinz which would have made her the first female player in Serie A.
However she did not end up with the club, which had also pursued Sweden's Hannah Ljungberg, prompting suggestions it had been a publicity stunt.