The 51-year-old was admitted to hospital in Madrid on Friday for tests and initial reports suggested that the five-time major winner had had an epileptic fit.
He was taken from the intensive care unit and underwent tests, which revealed that the 1997 Ryder Cup-winning captain is suffering from a brain tumour.
The Spaniard will undergo a biopsy on Tuesday to determine the course of action to be taken and he has reassured fans that he will fight the illness with the love and good wishes of his fans behind him.
He said: "Throughout my entire career I have been one of the best at overcoming obstacles on the golf course.
"And now I want to be the best, facing the most difficult game of my life, using all my strength and also counting on those who have been sending me get-well messages.
"Now I've been able to tell my three children personally and their mother, now I can tell you the illness that I am suffering from. After the exhaustive tests that I have undertaken in the Hospital La Paz they have detected a brain tumour."