The controversial Ultra Thai marathon race never received permission to operate in the fragile Chiang Dao Wildlife Sanctuary in Chiang Mai, said sanctuary chief Somsak Thithichayaporn.
The planned 150km trail run organised by French athlete Sebastien Bertrand never sought approval from the Department of National Parks, Wildlife and Plant Conservation or the wildlife sanctuary. Mr Somsak said he would not allow the event to be held in the forest area at its present scale.
According to the promotional campaign, up to 400 runners are planning to join the three-day ultramarathon.
Mr Somsak said 400 is too many and would threaten the sanctuary's environment, which can accommodate only 200 people at a time. The 20km route through the sanctuary includes areas not open to the public.
"If the organiser insists on inviting this number of participants and on this route, I will say no to approval. But if he decides to change the plan according to the rules, I may consider it," he said.
The Ultra Thai race planned for Dec 4-6 which would pass through the Chiang Dao Wildlife Sanctuary has drawn heavy criticism from villagers and nature lovers in Chiang Mai, who say the terrain cannot support thousands of people running and selling souvenirs all at once.
Environmental conservationist Rangsrit Kanjanavanit said he had talked to Mr Bertrand after learning about the event and said he would oppose holding the race in Chiang Dao.
Mr Rangsrit said he received a message from Mr Bertrand yesterday, who said he is thinking of alternative routes for the race.
The organiser has invited runners to join the Ultra Thai Chiang Mai (UTCM) races at a cost of US$495 (16,489 baht) per person. The goal is to reach Doi Luang Chiang Dao, which is the highest limestone mountain in Thailand and is situated inside the sanctuary.
The organiser, through his website and the Thailand Mountain Trail Facebook page, also invites people to register as volunteers and ambassadors for the event. They would help mark the trails before the races and assist with check-in.