Shawn Johnson, former Olympic gold medal winner, retires

Johnson wins gold at Beijing 2008

Former Olympic gold medal gymnast Shawn Johnson has retired after failing to recover from a knee injury in time to compete at London 2012.

The American, 20, won four Olympic and three World Championship medals in a career where her charisma as much as her talent marked her out as a star.

Johnson earned one gold and three silver medals at the Beijing games.

"It has become obvious that my left knee is not able to sustain the demands of gymnastics any longer," she said.

"All I can do now is gracefully retire and thank everyone who has believed in me and my journey.

Johnson on London 2012 in March

"I am announcing this now so the focus for the next three weeks will be on the incredibly talented gymnasts who are trying to make our 2012 Olympic team."

Johnson came to prominence when winning the gold medal in the all-around event at the 2007 World Championship.

She went on to win four medals in the 2008 Olympics in China, including gold on the balance beam.