Seve Ballesteros has suffered a "severe deterioration" in his neurological state, his family said today.
The Spaniard, a five-time major winner, had surgery on a brain tumour in 2008 followed by a course of chemotherapy.
A statement on http://www.seveballesteros.com this morning read: "The family of Seve Ballesteros can report a severe deterioration in his neurological condition."
Ballesteros, who announced his retirement from golf in 2007, collapsed at Madrid Airport in October 2008 and two days later came confirmation that he had a brain tumour.
He underwent an initial 12-hour operation, but further surgery was necessary before he was well enough to return home and begin chemotherapy and radiotherapy treatment.
"I am very motivated and working hard, although I am aware that my recovery will be slow and therefore I need to be patient and have a lot of determination," he said at the time.
"For these reasons I am following strictly all the instructions that the doctors are giving me. Besides, the physiotherapists are doing a great job on me and I feel better every day."
After a second course of chemotherapy at Madrid's Le Paz Hospital in February 2009 he said on his website: "The results of the check-up were really positive, better even than the first ones."
Two more courses followed and four months later Ballesteros made his first public appearance, saying it was "a miracle" to be alive.
In December 2009 he appeared on television to receive the BBC's Lifetime Achievement Award at the Sports Personality of the Year event.
He won the Open three times, the Masters twice and played an inspirational role in the Ryder Cup, helping Europe to lift the trophy in 1985, 1987, 1989 and 1995 before captaining them to another victory at Valderrama two years later.
Ballesteros turned professional in 1974 at the age of 16 and made his first huge impact two years later by finishing second in the Open alongside Jack Nicklaus at Royal Birkdale.
His first major title came in the 1979 Open at Royal Lytham, he then became Masters champion in 1980 and 1983 and lifted the Claret Jug again at St Andrews in 1984 - his greatest moment really - and back at Lytham in 1988.
After a total of 87 tournament wins, his retirement came following years of battling an arthritic back and knee problems.
He was planning a farewell appearance for British fans at last year's Open at St Andrews - not in the main event, but in the four-hole Champions Challenge - but was not well enough to travel.
Only last month Phil Mickelson decided on a Spanish menu for the Champions Dinner at The Masters in Augusta in honour of Ballesteros.