Nibali (Astana) went on to win the 197km stage from Saint Etienne to the summit finish at Chamrousse, beating Poland’s Rafa Majka (Tinkoff-Saxo) by 10s and Leopold Konig (Netapp-Endura) of the Czech Republic by 11s.

Meanwhile, Spaniard Alejandro Valverde (Movistar) finished in fourth place at 50s after jumping away in the last metres to beat Frenchman Thibaut Pinot (FDJ.fr) who was fifth at 53s.

Porte, whose demise began with 12.4km of the 18.2km climb to go, crossed the line in 27th place at 8m 49s and riding in the slipstream of his two teammates Spaniard Mikel Nieve and Welshman Geraint Thomas. He is now 16th overall at 11m 11s

Nibali’s stage win - his third of this year's Tour - has given him an overall lead that, barring disaster, is likely to assure him the overall victory when the Tour finishes in Paris on Sunday week

He now leads overall by 3m 37s over Valverde who rose from third to second place and by 4m 24s over Frenchman Romain Bardet (Ag2r) who was fourth when the stage began. Pinot is fourth at 4m 40s.

As soon as Porte crossed the line he rode straight to his team bus, dismounted and then entered it to recover. However, to his credit, he came out later to speak briefly to the awaiting media.

“I don’t think I dealt with the heat very well,” Porte said. “It’s one of those things.

“It’s a massive shame but we’ll see what happens tomorrow [Saturday]

“I feel more for my team-mates who have been brilliant for me every day.

"If it happens to me it can happen to other guys too. We’ll just keep on pushing.”

Sky team principal, David Brailsford said he was at a loss to pinpoint exactly what went wrong: “No doubt at the bottom of the climb today he felt the heat, but I don't think that we can give any excuses today, it was the same for everybody.

“You have your good days and bad days, and when the bad days come you just have to keep going on, and you go back and try to get another good day.”

Brailsford said he did not think hydration was the issue, but that he believed Porte and the team could regroup for Saturday’s 14th stage from Grenoble to Risoul in the Alps.

The stage includes three huge mountains - the 34km long Col du Lauteret, the 19km Col d'Izoard to the maximum altitude for the Tour of 2,360m and the 12.6km ascent to the finish at Risoul.

“There wasn't a lack of hydration, but that's something to consider,” Brailsford said of Porte ride on Friday.

“I don't know why happened, that's the honest answer, but obviously it wasn't the Richie that we've seen for the first part of the race.

“He just has to reset and recover and try to go again tomorrow.”

Asked what Sky’s goals would be now in a Tour that soon met problems with their defending champion Briton Chris Froome stopping in stage five due to injuries from a crash, Brailsford said: “We are not going to role over, it was a blow losing Chris.

“We came here to try to win the race. Now seeing how Nibali is going, it will be an interesting race, that's for sure.

“We re-calibrated to our ‘Plan B’ and now we have to re-calibrate again.

“You just have to take stock and not get to downbeat.”

Asked if the podium was possible for Porte even though he is 6m 47s behind third – that being Bardet at 4m 24s - Brailsford said: “I don't know. We have to see about the next two days and we can look to animate the race as much as possible.”