Leading Light provided Aidan O'Brien with a record sixth Gold Cup at Royal Ascot in a thrilling finish to the Group One.
The St Leger winner was backed as though defeat was out of the question but he was made to scrap for every inch of the two and a half mile contest by the two gallant mares that filled the places.
Fellow Irish raider Missunited had made almost all of the running with Royal Diamond and Brown Panther in her slipstream with Estimate buried in the pack and Leading Light on the outside in midfield.
Jim Crowley wound it up on board the leader turning into the straight as Ryan Moore had to bide his time for a run on last year's winner Estimate who was having her first run of the season.
The Queen's mare had brought the house down at the Berkshire track when prevailing 12 months ago, the first winner of the Royal Ascot feature to be owned by a reigning monarch in the history of the great race, but her preparation hadn't gone smoothly.
As Moore looked for a run, the eventual winner and Joseph O'Brien refused to allow him out forcing Estimate to switch to the inside.
She then came outside Missunited and inside Brown Panther who edged right into Leading Light who momentarily looked as though he would hang across the track.
O'Brien soon had him back on a straight line and he ran on gamely under strong pressure to win by a neck from Estimate with Missunited a short-head away in third.
The stewards enquired into the jockey's use of the whip and they found that he had used it above the permitted level and banned him for seven days.
He is scheduled to miss July 3-9 but could also be sidelined on July 10-11 having picked up two more days for the same offence in the last race.
July 5 is the date of the Coral-Eclipse, in which Derby winner Australia is a possible starter should he miss the Irish Derby. He was also fined £3,000.
Crowley also got four days for the same reason (July 3-7) for his ride on Missunited.
Immediately after the race Joseph O'Brien said: "When you win it is always a good ride, there's no such thing as a bad winning ride. I kept a straight line, Ryan (Moore, on Estimate) was looking for a bit of room but I was entitled to keep a straight line.
"He had a little look when he got to the front and then went a bit to his left, he's a big, lazy horse but I think he's better at a mile and six.
"I was rowing away on him but I had loads left, I was trying to hold off asking for everything for as long as I could. Ryan gave me a bit of help by coming up my inside as he pushed me along a bit - this fellow is as tough as nails."
Aidan O'Brien, winning the race for the sixth time, said: "He's idle, but he was in a lovely position and settled well. He was very lazy when he got there. Joseph was trying to keep him with company.
"We were worried about two and half miles as he's out of a Queen Mary winner. He's a horse we thought could go back to a King George maybe, but he was up there for the last half a mile and after two miles you never know what is going to happen.
I'm so lucky to have the horses and work with the people that I do, I'm in a very lucky position and the lads have unbelievable horses with unbelievable pedigrees and it (Ballydoyle) is an unbelievable place to train from."
John Warren, the Queen's racing advisor, said of Estimate: "It was tremendous, she was so brave and the Queen got such pleasure from it. Ryan said it was a career-best. Having no prep, everything was against her so to run so well was tremendous.
"Last year we were worried about very soft ground. Thankfully we didn't have that problem this year. If she comes out of this race well and the Queen wants to run her then she will. That was a hard race. The horses were out of their comfort zone.
"There is a cliche about teamwork, but this was a team effort to get her here. To nearly do it without a prep was a fantastic effort.
"The strange thing was last year no one clapped in the Royal Box. They all just said well done to the Queen. Everyone clapped this year afterwards. That says it all.
"There was a real buzz because she'd done so well. Prince Philip said 'my goodness that was a good race'. It was great. A real thrill all round."
Missunited's trainer Michael Winters said: "She's such a game mare and she never ceases to amaze me. Jim gave her a great ride. We'll keep her going now. I've got Galway on my mind, but I'll talk to the owners first."
Owner Dan Hutch said: "Before the race we wondered what we were doing here, but that finish brought the crowd to their feet."
Michael Owen's Brown Panther travelled as well as anything into the straight and was the first to challenge Missunited but failed to see out the trip, finishing fourth.
Owen said: "He has been beaten by better horses but has still made us immensely proud. He always tries his best and has the heart of a lion. This was his big target for the first half of the season.
"The Melbourne Cup is an option, so is Goodwood and the Irish St Leger. There's also the race here on Champions Day to think about it."
Sky Bet installed the winner as favourite to retain his crown in 2015 and spokesman Michael Shinners said: "He made hard work of it but you couldn't help be impressed by his guts and determination to win when he looked beaten at least twice.
"We go 3/1 for next year's race."