Outsider Anmaat stars on British Champions Day
Calandagan started 6-4 favourite for the Champion Stakes but drawn on the inside rail suffered severe traffic problems in the straight.
He barged his way to the front close home but Anmaat, under Jim Crowley, conjured up a searing turn of foot to overtake the Aga Khan's classy colt and land the 2000m showpiece by half a length at 40-1.
Crowley, who punched the air in triumph crossing the line, said: "He wasn't going to run after he disappointed in France but this was his day.
"He had a lovely position then we just had nowhere to go. I can't say what I was thinking but once I got the gap he went."
Francis Graffard, Calandagan's trainer, shrugged: "Not much to say, the straight was complicated."
The well-fancied Economics, trying to add this to his Irish Champion Stakes success last month, had to settle for sixth.
Earlier, Charyn prevailed after a gruelling duel with Facteur Cheval in the Queen Elizabeth II Stakes to confirm his standing as Europe's best miler.
Sent off 13-8 favourite after wins at Royal Ascot and the Prix Jacques Le Marois in Deauville, Charyn hit the front down the straight.
Facteur Cheval loomed up on his outside to deliver the coup de grace for France but Charyn pulled out a little extra under Silvestre de Sousa, crossing the line two lengths clear to the roar of the packed stands.
After being presented with the winner's trophy by British royal Princess Anne trainer Roger Varian said: "He's the finished article, he's fully developed and in his peak. Hasn't he been great all year? It's a great result.
"It's already been announced that he'll retire at the end of the season.
"We'll see how he comes out of this but we'll contemplate one more international run."
Facteur Cheval, repeating his second in the race 12 months ago, lost nothing in defeat, with his jockey Maxime Guyon commenting: "No excuses, he ran a great race.
"I waited as long as possible to challenge, but you have to push the button sometime!
"Unfortunately Charyn got away, but my horse was magic, he accelerated well, frankly he's incredible."
'Special' Kyprios
Out of luck in the big two races, Irish master trainer Aidan O'Brien did not leave Ascot empty handed, his superstar stayer Kyprios adding another chapter to his storied career in the Group Two Long Distance Cup.
Just a fortnight after notching up his eighth Group One at Longchamp the 8-11 favourite scooped this Group Two prize in his stride for his seventh win of a remarkable season.
With the mud flying on the rain-softened ground Kyprios, despite being on the go since April, looked a picture in the paddock and also on the track where under Ryan Moore he easily held off the challenge of Sweet William at the end of the 3100m stamina test.
O'Brien, who at Longchamp hailed Kyprios as "the horse of a lifetime", said: "What can you say, the people around him made it happen. He's a very special horse and something we've never seen before.
"He's relentless and has an unbelievable amount of quality. It doesn't matter whether you go slow or fast, he just goes there and switches off. Ryan rides him incredibly well and knows him inside out."
A third Ascot Gold Cup is the target for Kyprios in 2025.