Britain's Geraint Thomas is set to win the Tour de France after maintaining his overall lead on the penultimate stage of the race, a 31km time trial.
The Team Sky rider, 32, finished third behind Tom Dumoulin but still leads the Dutchman by one minute, 51 seconds.
Tour convention dictates that the race leader is never challenged on the final stage, so Thomas only needs to reach the finish in Paris on Sunday to win.
"It's insane. Tour de France! I can't believe it," said Thomas on ITV4.
"I'm really trying not to cry, it's unbelievable. I tried not to think about it, just take it day by day. I've won the Tour de France man, I don't know what to say."
Dumoulin beat defending champion Chris Froome by one second to win stage 20.
Froome's second place in the time trial means the four-time Tour champion climbs to third overall, leapfrogging Slovenia's Primoz Roglic.
However, his hopes of winning a record-equalling fifth Tour title were effectively ended in the Pyrenees mountains on stages 17 and 19, as his team-mate Thomas proved he was the man to beat.
It will be a sixth British and Team Sky victory in seven years in the three-week race and Thomas will be the first Welshman to win one of cycling's three Grand Tours.
He will also become the third Briton to win the Tour de France, after Sir Bradley Wiggins in 2012 and Froome.
A wobble - then Thomas chilled out
Thomas set out hard in the time trial and, despite a wobble when he locked up his back wheel, led by 14 and then 13 seconds at the two intermediate time checks.
However, he eased off in the closing stages, after a word on his race radio from Team Sky sporting director Nico Portal.
"When I had a wobble on that corner I thought, 'chill out a bit'," said Thomas. "Nico was on the radio and told me to make sure I win the Tour, don't risk anything.
"I didn't think the course was too technical but then every corner seemed like a 180 degree turn. I felt good and started putting the power down but then took every corner super slow.
"I can finally let the emotion come."
- Listen: BBC Radio 5 live's commentary of the closing stages
- Listen: BeSpoke at the Tour: Glory for Geraint
- Read: The making of Thomas - a rise from humble beginnings
- Read: Relive the stage 20 time trial
Thomas' Tour pedigree
This victory will come in his ninth Tour, one fewer than the record for most appearances before winning, held by 1980 winner Joop Zoetemelk of the Netherlands.
Thomas first rode in the Tour in 2007, when he was 140th of the 141 finishers.
But, like many British riders, he raced on both the track and the road in the early part of his career, winning three world and two Olympic team pursuit titles on the track between 2007 and 2012.
He joined the fledgling Team Sky in 2010 and was given a freer rein in one-day races but has mainly been used as a domestique - a rider who sacrifices his own chances in the service of a team leader - for Froome in each of his four Tour victories, and before this year his best finish had been 15th, twice.
His own Tours have not been without incident.
In 2013, he broke his pelvis after crashing on the opening stage, but managed to complete the race and help Froome pick up his first Tour title.
In 2015, he ended up in a ditch after crashing head first into a telegraph pole on a descent on stage 16. He lost just 38 seconds to stay sixth overall but struggled on a climb on stage 19, losing more than 22 minutes to drop down the standings.
And last year, after winning the opening individual time trial to become the first Welshman to wear the yellow jersey, he had to abandon the race when he crashed on stage nine and broke a collarbone.
In 2014, which started in Yorkshire, he ended up being the only Briton to finish the Tour, in 22nd, after Mark Cavendish crashed out on stage one and Froome pulled out on stage five.
The stages that defined Thomas' victory
The Welshman went in to this year's race saying he was hoping to challenge his team leader Froome.
Before the Tour started, he told BBC Sport: "The team have said that with the way I've been riding they're confident to give me that role of a backup guy and to race at least until the first rest day [after stage nine]."
He was second after stage nine and took hold of the race leader's yellow jersey on stage 11.
- Stage three: Team Sky finish second in the team trial to propel Thomas up the standings to third overall, three seconds adrift of race leader Greg van Avermaet.
- Stage six: A tactically aware Thomas picks up two bonus seconds near the finish to move himself up to second overall.
- Stage 11: Thomas attacks with 6km remaining on the final ascent to the summit finish at La Rosiere in the Alps, to finish 20 seconds ahead of Dumoulin and Froome and take the race leader's yellow jersey.
- Stage 12: Another late surge sees Thomas become the first British rider to win on the fabled Alpe d'Huez as he again leaves Dumoulin and Froome in his wake to cement his position as a real threat in the race. "There wasn't a chance in hell I was going to win," he said. "I just kept following Dumoulin and Froome. Can we just go to Paris now?"
- Stage 17: Into the Pyrenees and an attack in the closing few hundred metres sees Thomas finish third to put another nine seconds into Dumoulin as Froome falters on the final climb, finishing 48 seconds behind his team-mate.
- Stage 19: The final stage in the mountains and Thomas follows the attacks of all his rivals before sprinting to second on the stage to pick up more bonus seconds and move two minutes, five seconds clear. He has accrued 33 bonus seconds, 21 more than Dumoulin.
- Stage 20: Dumoulin wins the time trial, beating Froome by one second but Thomas, knowing he has plenty of time in hand, rides carefully around the final few bends to finish third on the stage but maintain a lead of one minute, 51 seconds.
What happened to Froome?
Froome was heavy favourite to win a record-equalling fifth Tour de France title and become the fifth man to do so. He came into the race as defending champion and holder of all three Grand Tour titles, having won the Vuelta a Espana last September and this year's Giro d'Italia in May.
But his hopes of also matching Eddy Merckx's record of four consecutive Grand Tour victories were ended in the Pyrenees mountains in the final week.
Froome normally gets stronger as the race progresses but he has never ridden the Giro in the same year as the Tour and the 3,000 extra kilometres in his legs seemed to leave him jaded, particularly on stage 17.
That stage was only 65km long, the shortest of the modern era, but featured three brutal ascents, the last of which reached an altitude of 2,215m - the highest point in the race.
Froome fell off the pace and had to be nursed home by 21-year-old team-mate Egan Bernal, and the Colombian also had to help Froome stay with the leaders on stage 19, although he lost third place to Roglic.
However, he rode a superb time trial on Saturday to jump back above the Slovenian and ensure there will be two Britons on the podium in Paris on Sunday.
Stage 20 result:
1. Tom Dumoulin (Ned/Sunweb) 40mins 52secs
2. Chris Froome (GB/Team Sky) +1sec
3. Geraint Thomas (GB/Team Sky) +14secs
4. Michal Kwiatkowski (Pol/Team Sky) +50secs
5. Soren Kragh Andersen (Den/Sunweb) +51secs
6. Bob Jungels (Lux/Quick-Step Floors) +52secs
7. Ilnur Zakarin (Rus/Katusha-Alpecin) +1min 02secs
8. Primoz Roglic (Slo/LottoNL-Jumbo) +1min 12secs
9. Marc Soler (Spa/Movistar) +1min 22secs
10. Michael Hepburn (Aus/Mitchelton-Scott) +1min 23secs
General classification after stage 20:
1. Geraint Thomas (GB/Team Sky) 80hrs 30mins 37secs
2. Tom Dumoulin (Ned/Team Sunweb) +1min 51secs
3. Chris Froome (GB/Team Sky) +2mins 24secs
4. Primoz Roglic (Slo/Team LottoNL-Jumbo) +3mins 22secs
5. Steven Kruijswijk (Ned/Lotto NL-Jumbo) +6mins 08secs
6. Romain Bardet (Fra/AG2R La Mondiale) +6mins 57secs
7. Mikel Landa (Spa/Movistar) +7mins 37secs
8. Daniel Martin (Ire/UAE Team Emirates) +9mins 05secs
9. Ilnur Zakarin (Rus/Katusha-Alpecin) +12mins 37secs
10. Nairo Quintana (Col/Movistar Team) +14mins 18secs