Cycling chiefs on Tuesday requested that the World Anti Doping Agency (Wada) "take a position" on the controversial but legal carbon monoxide breathing method used by top riders.
Tour de France champion Tadej Pogacar and two-time winner Jonas Vingegaard are among those known to have practiced the method believed to effect the level of red blood cells in the bloodstream.
"The UCI clearly asks teams and riders not to use repeated CO inhalation. Only the medical use of a single inhalation of CO in a controlled medical environment could be acceptable," the International Cycling Union (UCI) said on Tuesday.
"The UCI is also officially requesting the World Anti-Doping Agency (Wada) to take a position on the use of this method by athletes."
The use of the potentially lethal gas by at least three teams was revealed last summer during the Tour de France by the specialist cycling website Escape Collective.
Among these teams are Israel PT, Pogacar's UAE Team and Vingegaard's Visma.
Pogacar was asked about the use of carbon monoxide and seemed unaware of the method before accepting a day later he had done it on one occasion under medical supervision.
"It's a device to test how your body reacts to altitude," Pogacar, a three-time Tour de France winner, explained.
"We blow into a balloon for one minute for a test that we have to do two weeks apart. I only did the first part because for the second, the girl who was supposed to do it never came. It's not like we breathe that every day."
The method is not yet illegal but given repeated use can create an effect similar to altitude training Wada are likely to make some sort of ruling on its use.