Giant's Causeway put the disappointment of two Guineas defeats behind him when striding to victory in a thrilling St James's Palace Stakes which lit up the opening day of Royal Ascot.

In a breathtaking finish to the first Group One event of the meeting Aidan O'Brien's charge defied the strong late challenge of Valentino to land the £156,600 first prize by a head with Medicean a further three-quarters of a length back in third.

In a change of tactics winning jockey Michael Kinane chose to attempt to force the pace on the 7-2 favourite and the ploy was working to perfection until Valentino loomed up alongside.

In fact, Gerald Mosse forced the eventual runner-up to the front but the partnership failed to repel the renewed charge of Giant's Causeway in the shadow of the post.

Having seen their charge finish second to King's Best in the Sagitta 2000 Guineas at Newmarket and fill the same position behind Bachir in the Irish version of the Classic at the Curragh, Gaint's Causeway's success came as a great relief for connections.

Kinane felt that the Storm Cat colt was unlucky in both races.

''I was hard on him at the Curragh,'' said the jockey. ''And tactically I got it wrong so I wanted to make amends today."

At the same meeting the Aga Khan secured a 285-1 treble with Kalanisi, Dalampour and Katiykha.

But the victories would have rubbed salt into the wounds of the absent Luca Cumani who trained all three horses last season before being sacked by the owner.

Kalanisi and Dalampour are now in the care of Sir Michael Stoute while Katiyhka is trained by John Oxx.

''This is a day not to forget and not to expect,'' said the owner-breeder. ''The horse I was most focused on was Dalampour but they've all done us proud.''

The victories were continuing a successful spell for the Aga Khan, who earlier in the month landed the Vodafone Derby with Sinndar.

The owner has plenty more to look forward to this week, beginning with Sendawar taking on Godolphin superstar Dubai Millennium in today's Prince of Wales's Stakes.