Porte lost contact with Italian race leader Vincenzo Nibali and his other overall rivals and was then distanced to finish more than eight minutes behind.

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

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

Nibali's stage win 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.

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 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 well see what happens tomorrow.

"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 today's 14th stage from Grenoble to Risoul in the Alps.

Asked if the podium was possible for Porte even though he is 6:47 behind third - that being Bardet at 4:24 - 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."