Swiss teenage cyclist Muriel Furrer (18) is in a "very critical condition" after crashing heavily during the women's junior road race at the world championships in Zurich on Thursday.
"Muriel Furrer was taken to hospital by an emergency helicopter. She suffered a serious head injury and is in a very critical condition," the UCI, cycling's governing body, said in a statement.
Cycling officials said they were "extremely concerned by the situation".
"At present, there are no established facts regarding how the accident happened. Investigations by the competent authorities are ongoing," the UCI added.
The race took place in wet conditions as rain fell in Zurich throughout Thursday.
Furrer won silver in both the junior time trial and road race at this year's Swiss nationals.