Fox, 38, has announced he will leave the political comedy at the end of its current season.

The Back to the Future star said: "I could not be more proud of the show ... and all that we have accomplished over the last four years.

"But I feel that right now my time and energy would be better spent with my family and working towards a cure for Parkinson's Disease."

The announcement came just days before the Golden Globes awards ceremony, where he has been nominated as best comedy actor for his Spin City role.

'Winnable war'

Last year, Michael J Fox visited Washington to call for urgent extra funding into the condition.

He told a Senate committee: "The war against Parkinson's is a winnable war and I have resolved to play a part in that victory."

Fox revealed in 1998 that he had been suffering from Parkinson's since 1991. The condition was diagnosed after he noticed a twitch in his little finger while he was working on the set of the film, Doc Hollywood.

Parkinson's is a degenerative disease of the nervous system that generally affects both men and women who are more than 40-years-old. However, like Fox, 10% of sufferers are believed to be under 40.

The disease develops slowly over time and is associated with trembling of the limbs, stiffness of the muscles and slowness of movement.

Fox found fame in the Back to the Future films after starring in the TV sitcom Family Ties.