David Knight has been forced to pull out of the International Six-Day Enduro.
Doctors have advised him to rest due to ongoing problems following his crash at the World Championship round in Finland on September 1.
After spending a night in hospital following the incident, Knighter flew to Manchester for further tests.
Doctors say he has issues with low red blood cell count resulting in a lack of oxygen in his blood.
He has been advised to rest while further tests take place to find the cause.
The ISDE takes place between 24th and 29th September and Knighter was due to represent the Great Britain World Trophy team.