The crash involved two motorbikes and 19 riders, a spokesman for the race told Reuters. Five of the riders were taken to hospitals near the crash, which took place near Verviers, close to the border with Germany.
"This afternoon Stig Broeckx got seriously hurt in an accident during the third stage of the Belgium Tour," the team said in a press release.
"In a descent the young Belgian was involved in a crash of about twenty riders, caused by a crash of two motorbikes. Broeckx was seriously injured and he immediately received medical attention at the side of the road, afterwards a helicopter transported him to the hospital of Aachen.
The stage was immediately neutralised after the crash.
“It was immediately clear that Stig had incurred a head injury and that he needed to be transported to a neurosurgical centre," team doctor Servaas Bingé said.
"Together with the race doctor and the medical emergency team we decided to transport him to the hospital of Aachen, which has the necessary infrastructure. A scan was taken at the hospital and it showed that Stig has two bleedings in his brain.
"Stig is in a non-induced coma, but for the moment he doesn’t need to undergo surgery. He doesn’t have a skull fracture, but he does have a fractured eye socket, which will be further evaluated later. At this point, it is very difficult to give a prognosis about a full recovery. The next 24-hours he will be very closely monitored.”
The accident is sure to trigger new controversy over the safety of cycling races. In March, Belgian rider Antoine Demoitie died after being hit by a motorbike during the Gent-Wevelgem one-day race, the latest in a string of incidents involving race vehicles over the last year.
“After the neutralisation, the riders gathered in the team bus and drove to the team hotel were they stayed together to support each other and to talk about what happened," Lotto Soudal manager Marc Sergeant said.
"After deliberation we informed the organisation that we won’t ride the last stage of the Belgium Tour. At the moment it is pointless to make any accusations, but it should have been obvious much longer that structural measures concerning the safety of the riders are necessary.”