BURGHLEY HORSE TRIALS
Venue: Burghley Date: 3-6 September
Coverage: Live on BBC Two and BBC Sport website 1400-1530 BST on Sunday
King was part of the British Euro gold medal-winning team two years ago
Great Britain's hopes of defending their European eventing title in France later this month suffered a blow after Mary King's horse had to withdraw.
King was part of the British European gold medal-winning team two years ago.
But her ride Imperial Cavalier suffered an injury and will now not be risked with the 2010 World Equestrian Games and London 2012 Olympics coming up.
The news came at the Burghley Horse Trials where defending champion William Fox-Pitt finished 13th on day one.
The British rider, onboard Macchiato, made an uninspiring start in the dressage accruing 55.7 penalties.
Fox-Pitt is chasing a record sixth Burghley title and a hat-trick of victories, after tasting glory in 2007 and 2008.
Victory at Burghley would boost Fox-Pitt's hopes of landing the HSBC Classics title, which carries a £91,500 top prize.
Burghley is the fourth of five events in the series after Lexington, Badminton and Luhmuhlen, with Fox-Pitt currently one point behind leader and GB team-mate Oliver Townend.
The event, which began with dressage tests on Thursday included four of the six British riders selected for the European Championships in France this month before King's withdrawal with Nicola Wilson the other member.
The two members not involved at Burghley are Tina Cook and Lucy Wiegersma.
Most attention revolves around Fox-Pitt with GB team-mate Townend, who won at Badminton on Flint Curtis, his main rival and he rides Carousel Quest.
Both riders are in action on Friday, with Fox-Pitt returning to the dressage arena on Seacookie, while 2009 Badminton winner Townend launches his Burghley challenge.
American Phillip Dutton is the early leader, recording 45.3 penalties onboard Truluck in their dressage test during an opening day of testing windy conditions.
Half the 80-strong field were in dressage action on Thursday, with the remaining 40 competitors participating on Friday.
Leicestershire rider John-Paul Sheffield (Crown Farm Consort) is in second place, with Germany's Kai Ruder (Leprince Des Bois) third.
Fox-Pitt has entered two horses - Macchiato and Seacookie - as the build-up towards Fontainebleau continues.
British rider Zara Phillips, who was pleased with Toytown's post-injury run at Highclere in Berkshire last weekend, takes Glenbuck to Burghley and features on Friday.
After dressage on Thursday and Friday, attention switches to a 27-fence cross-country course - designed by Captain Mark Phillips - on Saturday, before Sunday's show-jumping concludes the competition.