Phillips was denied the chance to compete at Athens 2004 and Beijing 2008 after her horse Toytown succumbed to injury.

Grandfather Prince Philip, the Duke of Edinburgh and her mother Anne, the Princess Royal lent support to Phillips and attended her Olympic debuting event.

Phillips, taking part in the dressage at Greenwich Park, is following in the footsteps of her mother who competed at Montreal in 1976.

“It's incredible to be selected for the team and to be here at home in London is an amazing feeling and I just want to do my best for the team,” Phillips said.

On her horse's performance she said, “This was High Kingdom's second biggest test, after the Burleigh Horse Trials, he's only getting better.”

“He's a good jumper, quick to turn and he's bored with dressage now and wants to get out there. Although he's inexperienced he's coped with the crowds well. He is a chilled character.” she added.

Phillips said, “The Olympics is the greatest show on Earth and it is incredible to be here and to be competing for my country.”

In the first day of the dressage competition Saturday, Australia took the early lead with Germany and the United States close behind.

Phillips is now looking forward to the second leg of the eventing competition, the cross country, on Monday.

Great Britain is seeking their first equestrian gold since Munich 1972 with Mark Phillips, Zara's father, who is at London 2012 as manager of the United States squad, on the team.