Hot favourite Leading Light narrowly denied the Queen's defending champion Estimate in a pulsating climax to the Gold Cup at Royal Ascot.

Last year's Queen's Vase and St Leger hero Leading Light was all the rage for the two-and-a-half-mile Group One, having made a winning return to action in the Vintage Crop Stakes at Navan last month.

The four-year-old was under the pump from the home turn in the hands of Joseph O'Brien, son of trainer Aidan, but stuck to his guns admirably to fend off Estimate by a neck, with the front-running Missunited a gallant third.

Estimate 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.

Her preparation for the defence of her crown did not go smoothly and as a result, Sir Michael Stoute's mare was making her first start since October.

The five-year-old travelled powerfully for a long way and appeared to have most of her rivals covered on the home turn, but crucially, she was kept in by Leading Light and had to go for a run up the inside.

When the gap came, Estimate fought on bravely, but 10-11 favourite was just too strong.

Last year's Galway Hurdle heroine Missunited was a 40-1 shot taking a significant step up in class and benefited from a superbly-judged ride from the front by Jim Crowley.

She appeared to have cut loose shortly before the turn for home, but although she gave her all in the closing stages, she was eventually run out of it and had to make do with minor honours, just a short head further back.

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."