Heavy snow forced Giro d'Italia organisers to call off today's mountainous stage 19 from Ponte di Legno to Val Martello and the route for the next day's stage was re-designed.
Tomorrow's stage includes far fewer descents to ensure another day of mountain racing is not lost but it will still finish on the Tre Cime di Lavaredo at 2304 metres above sea level.
"Descents on mountains that are covered in snow are the worst thing for the riders," the race's technical manager Mauro Vegni told a news conference at today's cancelled finish of Val Martello.
"If it's snowing, we'll clear the roads. Knowing how cold it will be is essential. Snow is cold, but it's the descents that really matter."
Three classified climbs through the Dolomites have been annulled in tomorrow's redesigned 210-km stage and only two short descents near the final climb of Tre Cime di Lavaredo are still in place.
"The riders have promised me personally to help us as much as they can and that they will complete the course. That was very considerate of them," Vegni added.
As the snow fell increasingly heavily on to the ground which was already covered with a layer several centimetres thick, Vegni was asked what the weather forecast was for tomorrow.
"Look out of the window," he answered abruptly.
"But we've got people at key points to warn us if it all becomes impassable. But in nearly 25 years of working on the Giro, I've never known a day like this one."
Poor weather had already led race organisers RCS Sport to make changes to the stage-19 route on Thursday, cancelling two of the three major mountain climbs, the Gavia and Stelvio, but they had hoped to retain the final ascent to Val Martello.
However, major overnight snowfall, temperatures well below freezing on the major mountain passes and sleet falling in the region's valleys early on Friday caused the entire stage to be cancelled, the first time that had happened since 1989.
Overall leader Vicenzo Nibali applauded the organisers' decision, saying at his team hotel: "The conditions are really extreme and so I think they've made the right decision.
"They'd already altered the stage route but because of the weather conditions and the snow we can't do the stage," added the Italian, who leads by more than four minutes ahead of Australia's Cadel Evans and Colombia's Rigoberto Uran.
The last Giro stage to be cancelled like Friday's for reasons purely to do with the terrain or weather was 24 years ago, when a landslide, possibly caused by heavy snow, blocked one mountain pass.
"It's a strange and almost unique day in the history of the race," race director Michele Acquarone told the news conference. "We had to cancel today's stage because there was no way that we could have a race."
With rainstorms, cold, thunderstorms and now heavy snowfalls plaguing the Giro since stage four, last Saturday's ascent of the mid-stage Col de Sestiere was cancelled, and on Sunday the finish of the final ascent of the Col du Galibier in France was held four kilometres from the snow-covered summit.
The race, which is due to finish in Brescia on Sunday, was rocked on Friday by the news of former winner Danilo di Luca's positive test for the banned blood booster erythropoietin (EPO).
The 37-year-old Italian, who won the Giro in 2007, failed an out-of competition test taken on April 29 at his home, five days before the start of this year's Giro. He had a previous positive for the same banned substance in 2009.
"He's an old athlete who hasn't understood the world has changed and that cycling is working to become a clean sport," Acquarone said.
"On a personal level, if somebody tells me 'I've made a mistake and I won't do it again', I believe them. If they then make the same mistake, I start thinking in terms of drug addiction."