Tao Geoghegan Hart became only the second British man to win the Giro d'Italia as he took the title in Milan.
The 25-year-old won Saturday's stage 20 to share the lead with Jai Hindley before the final stage, the first such instance in the race's history.
Ineos Grenadiers' Geoghegan Hart finished 39 seconds ahead of Hindley in Sunday's 15.7km time trial.
"Not in my wildest dreams did I imagine this would be possible when we started," said Geoghegan Hart.
"It's incredible, really incredible."
Chris Froome is the only other British man to win the Giro, in 2018.
Geoghegan Hart is the fifth British man - alongside Froome, Bradley Wiggins, Geraint Thomas and Simon Yates - to win a Grand Tour.
In only his fourth Grand Tour, Geoghegan Hart began the race as one of Geraint Thomas' domestiques and was in 126th place after the opening stage.
Geoghegan Hart described his victory as "bizarre", adding: "All of my career I've dreamed of trying to be top 10 - top five maybe - in a race of this stature, so this is something completely and utterly different to that and it's going to take a long time to sink in."
Who is Tao Geoghegan Hart?
Geoghegan Hart grew up in Hackney in east London, where he joined Cycling Club Hackney.
He is also a keen swimmer and at the age of 13 was part of a group of teenagers who swam across the English Channel.
As a fan, he attended an event in London to mark Team Sky's launch in 2010 and would later sign for the British team, competing full-time on the top-tier World Tour since 2017.
He has remained with the team, who became Ineos in 2019, ever since.
His previous best finish at a Grand Tour was 20th at the Vuelta a Espana in 2019.
After winning the Giro he shared a kiss with girlfriend Hannah Barnes, also a professional British cyclist.
"Tao bunked off school to come and ride behind the other guys [when Team Sky launched] and he has gone and won a Grand Tour," team boss Sir Dave Brailsford said.
"It is the stuff of comic books."
How the race was won
Geoghegan Hart was not the leading contender in his team, let alone the race, when the Giro began.
He took on the role of Ineos leader after Thomas, the pre-race favourite, pulled out with a fractured hip suffered in a crash on stage three.
Other expected contenders for the pink jersey, Yates and Steven Kruijswijk withdrew after testing positive for coronavirus over a remarkable three weeks which also featured team protests and withdrawals.
Geoghegan Hart won stage 15 - his first Grand Tour victory - but at that point was still almost three minutes off the overall lead.
He followed that with a stunning performance in the mountains, catapulting himself into contention with second place over the Stelvio Pass on stage 18 and winning a gruelling stage on Saturday.
The final stage time trial saw the riders setting off in reverse order with the Brit starting three minutes before Australian Hindley on Sunday, Geoghegan Hart was always ahead at the intermediate time checks and finished in 18 minutes 14 seconds.
He became the first Giro winner to have never held the leader's pink jersey until the final finishing line.
"My DS (director sportif) told me I was 10 seconds up and then he kept giving me a few seconds," he said.
"I only knew we must be in a pretty good situation when they were screaming not to take any risks in the last kilometre."
Ineos' Filippo Ganna won the stage by 32 seconds, his fourth stage win of the race.
With seven stage wins, it was Ineos' most successful Grand Tour since they were formed.
Wilco Kelderman, who lost the overall lead to Sunweb team-mate Hindley on stage 20, took third place overall, 1min 29secs down.
Joao Almeida, who led for 15 days, finished fourth and Pello Bilbao fifth.
Vincenzo Nibali was seventh, leaving Italy without a rider in the top five for the first time in Giro history.
Stage 21 result
1. Filippo Ganna (Ita/Ineos Grenadiers) 17mins 16secs
2. Victor Campenaerts (Bel/NTT Pro Cycling) +32secs
3. Rohan Dennis (Aus/Ineos Grenadiers) same time
4. Joao Almeida (Por/Deceuninck-Quick-Step) +41secs
5. Miles Scotson (Aus/Groupama-FDJ) same time
6. Josef Cerny (Cze/CCC) +44secs
7. Chad Haga (USA/Sunweb) same time
8. Brandon McNulty (USA/UAE Team Emirates) +46secs
9. Kamil Gradek (Pol/CCC) +47secs
10. Jan Tratnik (Slo/Bahrain-McLaren) same time
Final general classification
1. Tao Geoghegan Hart (GB/Ineos Grenadiers) 85hrs 40mins 21secs
2. Jai Hindley (Aus/Sunweb) +39secs
3. Wilco Kelderman (Ned/Sunweb) +1min 29secs
4. Joao Almeida (Por/Deceuninck-Quick-Step) +2mins 57secs
5. Pello Bilbao (Spa/Bahrain-McLaren) +3mins 9secs
6. Jakob Fuglsang (Den/Astana) +7mins 2secs
7. Vincenzo Nibali (Ita/Trek-Segafredo) +8mins 15secs
8. Patrick Konrad (Aut/Bora-Hansgrohe) +8mins 42secs
9. Fausto Masnada (Ita/Deceuninck-Quick-Step) +9mins 57secs
10. Hermann Pernsteiner (Aut/Bahrain-McLaren) +11mins 5secs
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