The ‘World’s coldest and toughest’ ultra marathon begins tomorrow, but not from Shipyard’s Park in Whitehorse as previously planned.

Arctic Ultra organizer Robert Pollhammer says warm weather has made the planned Yukon river route impassable.

“The most important one really is open water,” Pollhammer says. “That was difficult to get around and parts where there is gravel, and around Shipyards Park, of course, you can't go on the river really so you have to stay on land and that's not really great because of lack of snow.”

Parts of southern Yukon have been experiencing unusually warm weather for the past month, including a record high of 6.5 C in Whitehorse last week.

The start line for the Arctic Ultra has now moved to the Takhini Hotsprings, 30 km north of Whitehorse.

It follows changes this week to the Yukon Quest route.

The move means athletes running the marathon portion of the Arctic Ultra will run 20 km north, then come back to the finish line at the hot springs, and have an opportunity to take ‘a well-deserved and relaxing bath,’ says the race website.

Ultra-marathoners will not have that option. According to checkpoint rules, those running 160 and 480 km races are not allowed inside during the race — for them, it’s winter camping only.

Ultra-marathoners this year will also have the mental challenge of backtracking. All in all, they’ll have to run, bike or ski one 12 km portion of the route three times.

Fifty-three people from North America and Europe are registered to take part in the race.

Most will compete on foot, with a smaller contingent of bikers and skiers.