Swiss rider Muriel Furrer has sustained a serious head injury after she fell heavily during the women's junior road race at the world championships in Zurich.

The sport's governing UCI said the 18-year-old was air-lifted immediately to hospital by an emergency helicopter on Thursday but the reason behind her fall has not yet been determined.

"She suffered a serious head injury and is in a very critical condition," the UCI said in a statement.

"Extremely concerned by the situation, the Union Cycliste Internationale, Swiss Cycling, and the Zurich 2024 Local Organising Committee are closely monitoring developments and will communicate as soon as possible."

The UCI added there were no "established facts" regarding how the accident happened.

"Investigations by the competent authorities are ongoing," it added.

Furrer finished 44th in the time trial that was held earlier this week.