KTM and Tyla Rattray have 'mutually agreed' to part company, so that the 2008 MX2 world champion can fulfil his dream of moving to the AMA in 2009, rather than graduating - as intended - with KTM to the premier MX1 World Championship.
Rattray will thus follow in the footsteps of countryman and fellow KTM MX2 world champion Grant Langston, who moved to America after winning his world title and later experienced title success with Kawasaki and Yamaha.
It is rumoured that Rattray will ride for the multi-title winning Pro Circuit Kawasaki team in 2009.
"KTM and its factory rider Tyla Rattray have reached a mutual decision to terminate the South African's existing contract for the 2009 racing season so that Tyla may pursue his long-term dream of competitive riding in the USA," said a short KTM statement.
"The company, while still convinced that the MX2 World Champion's career would have been better served by remaining in the Motocross World Championship to contest the MX1 title in 2009, nevertheless respects his desire and wishes him well in the next phase of his racing career."
Rattray had been due to headline KTM's 2009 factory MX1 challenge alongside Max Nagl and Jonathan Barragan. Although it would have been his first season on a 450, he was considered a firm title favourite.
Rattray's 2008 team-mate and title rival Tommy Searle will also move to the AMA championships next season, although the young Briton will remain with KTM.
Greg Albertyn was the first South African to win both world championship and AMA motocross titles.