Bradley Wiggins struck a major blow in his quest to win the Tour de France by extending his lead over his main rivals during the final day in the mountains.
With three stages to go - including Saturday's time trial - his lead over Team Sky team-mate Chris Froome remains at two minutes and five seconds.
But third-placed Vincenzo Nibali lost 18 more seconds as Britons Froome and Wiggins finished second and third.
Alejandro Valverde held off a late assault by Sky to win in the Pyrenees.
"Barring accidents, that is definitely job done and the Tour won for Bradley Wiggins. He and Team Sky had as easy a day as they could have had hoped because they had to do virtually nothing. Near the end, Wiggins and Froome were probably saying 'we've done it' as much as anything because I don't think they would have expected Vincenzo Nibali to lose time. The only thing I regret is that if Froome had been given a free hand, he could have won the stage too."
Only two more days of real racing lie between Wiggins and the top of the podium in Paris.
The yellow jersey is, by convention, never attacked on Sunday's final stage, which is a procession into the French capital and to the finish line on the Champs Elysees, bar the bunch sprint for the stage win.
Wiggins was expected to be given a tough test on stage 17, the last day in the mountains.
Given the Briton is a superior time trialist, Nibali knew this was effectively his last chance to attack.
The Italian's Liquigas team tried to set up him up for a stage win, as well as putting pressure on Team Sky, on the climb towards the finish line at Peyragudes but he could not keep the pace all the way to the finish line.
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“If you had said to me a month before the Tour that with three days to go I would be second I wouldn't have believed you. I'm very happy”
It was left to Froome and Wiggins to try to chase down Spaniard Valverde, who eventually crossed the line first, 19 seconds ahead of the Team Sky duo.
Nibali is now two minutes and 41 seconds adrift of Wiggins, who admitted he lost concentration in the final section of the stage.
"We were talking about Nibali," said Wiggins. "We knew he was on his limit. The moment we crossed the Peyresourde, I allowed myself to drift and that was the first time I thought maybe I've won the Tour today.
"All the way up that last climb my concentration had gone, everything about performance had gone. Chris was egging me on to take more time and I was in another world, really."
Froome, who appeared keen to attempt to overhaul Valverde, played down suggestions he sacrificed a stage win to assist Wiggins.
"Everyone in the team makes sacrifices for the yellow jersey, that's cycling," he said. "It's our work.
"I'm 27 and I hope to win the Tour one day. If you had said to me a month before the Tour that with three days to go I would be second I wouldn't have believed you. I'm very happy."
- Friday's stage 18:
Bunch sprint expected at end of 223km ride
- Saturday's stage 19:
53.5km individual time trial. Wiggins' strong suit; he could extend his advantage
- Sunday's stage 20:
A procession for the man in yellow. Mark Cavendish chasing a fourth successive win on the Champs-Elysees
Wiggins added: "I'm sure that he will win the Tour one day. Yes, maybe he is stronger than me in the mountains, for sure, but I'm not a true climber. I'm still a rider against the clock who can climb."
Team Sky general manager Dave Brailsford praised the rest of his squad for their support of Froome and Wiggins but said he would not relax until the 32-year-old Londoner is sure of becoming the first British rider to win the Tour in its 109-year history.
"Credit to the guys they did it again," Brailsford said. "All the work they did to set it up for the two guys at the end was brilliant. Now that the mountains are over we can let out a sigh of relief and look forward to Friday.
"We set out to consolidate the lead and the yellow jersey and we showed again we are the best team in the race. Unity was really important to us, I'm very proud of that.
"The closer you are the more you have to lose. It is my job to ensure that tomorrow [Friday] we'll be as vigilant as at the start of the Tour."
While Wiggins and Froome consolidated their positions at the head of the race, world champion Mark Cavendish had to overcome a crash 3km from the finish after a flag waved by a spectator got wrapped around his handlebars.
Cavendish, who was reported to be "fine" by Team Sky sports director Sean Yates, escaped with a swollen leg.
Stage 17 top five:
Alejandro Valverde (Spa) Movistar 4:12:11"
Chris Froome (GB) Team Sky +19"
Bradley Wiggins (GB) Team Sky +19"
Thibaut Pinot (Fra) FDJ +22"
Pierre Rolland (Fra) Europcar +26"
Overall race standing after stage 17:
Bradley Wiggins (GB) Team Sky 78:28:02"
Chris Froome (GB) Team Sky +2:05"
Vincenzo Nibali (Ita) Liquigas +2:41"
Jurgen Van den Broeck (Bel) Lotto Belisol +5:53"
Tejay Van Garderen (US) BMC Racing +8:30"