Rickardsson retires.

Reigning six times world champion Tony Rickardsson, widely considered Speedway's greatest ever rider, has announced his retirement.

The 36-year-old Swede says that he has been forced to take the difficult decision after suffering numerous concussions - which are becoming ever harder to recover from.

"Mentally it is impossible to race, the body has taken too many hits," the BBC reports Rickardsson as saying. "I am afraid of receiving another concussion. Then I will not know what will happen."

Despite the announcement, Rickardsson - currently thirteenth in the 2006 championship standings - may say goodbye to his adoring fans with one final ride in his home grand prix on August 12.

Rickardsson won the world championship in 1994, 1998, 1999, 2001, 2002 and 2005, and achieved World Cup wins in 1994, 2000, 2003 and 2004.