Formula One ringmaster Bernie Ecclestone has warned the Australian Grand Prix is the “least viable” race on the calendar and could be under threat if it does not become a night event. The sport has a deal with Melbourne until 2015 but beyond that it is up in the air, with others cities lining up to take its place, he said. “We have a contract which we will respect — so up until 2015 we are in good shape,” Ecclestone was quoted as saying on Thursday by Fairfax Media ahead of Melbourne hosting the opening race of the season on March 18. “After then, we really don’t know. If we were to have a divorce from our friends in Melbourne we would probably be walking away from Australia.