Idaho could join his full-brother Highland Reel in the King George VI And Queen Elizabeth Stakes next month after winning the Hardwicke Stakes on the final day of Royal Ascot.
Just as his sibling had on Wednesday in the Prince of Wales's Stakes, the 9-2 chance proved too good for his rivals, beating Barsanti and Chemical Charge by half a length and a length and a quarter, as winning rider Seamie Heffernan timed it to perfection.
The Queen's Dartmouth had every chance when hitting the front seemingly full of purpose, but in the end had to settle for fourth.
The victory secured Aidan O'Brien top trainer honours at the meeting for an eighth time.
Idaho slipped up when odds-on favourite in the St Leger in September and was last seen finishing sixth behind Highland Reel in the Coronation Cup at Epsom.
O'Brien said: "We're delighted. He's a full-brother to Highland Reel and we always loved him. It was his first run of the year in the Coronation Cup. He only arrived at the track an hour before and he was very upset.
"He was very unlucky in the St Leger last year when he unseated Seamus, you couldn't believe it, he was coming down the straight on the bridle.
"He's by Galileo and they don't know when to stop improving or stop trying. It's very possible he'll progress, there's plenty of races for him. I think he gets a mile and a half well, he galloped out very well and those races are open to him.
"It (King George) is a possibility for him, the lads will decide what they want to do in a week or 10 days, but it's a very strong possibility.
"Seamus gave him a peach of a ride, he's a world-class rider. He timed his run to perfection."
On being crowned leading trainer, O'Brien added: "We're obviously in a very privileged position to be working with such people and horses. It's a lot of hard work by a lot of people day in and day out.
"I'm very lucky to be able to go home every night (from Ascot). We're at home at 8.30pm every night and it's great to be able to do it. We look forward to going home, seeing the horses in the morning and then come racing. It's marvellous really."
Barsanti's trainer Roger Varian said: "We are delighted with the horse and are very proud of him. It's a career-best.
"It is a bit disappointing that he ran so well and didn't win. I think the draw beat him. When he got into the straight he got kicked out even wider by Western Hymn.
"Well done to the winner, but I think our horse has run a mighty race. We will get him home and see how he is but he has got lots of options. We will try and get him back on the winning trail.
"I always thought last year he could be an Australian horse - as in a Cup horse. I think he has got that profile. He is a gelding that goes on fast ground and has got a touch of class. He is not slow either."
The third home Chemical Charge is trained by Ralph Beckett, who said: "It's a career-best. We thought the fast ground and the straight here would suit him. We will probably go travelling with him and I can see him running in America. Probably long term, something like the Canadian International might be for him."
Stoute made no excuses for Dartmouth, saying: "I thought it was going well until they got serious. He was just not good enough today."