Lyle, 36, underwent a bone marrow transplant in December following a recurrence of acute myeloid leukaemia last year.

He has twice beaten cancer, in 1998 and 2012, and been able to return to play professional golf.

Lyle's wife Briony has posted another update following an entry on his website this month that described his battle as "the longest and hardest one so far".

"His condition is a complete mystery, so it's just a waiting game while they run several tests, compare results and get specialist input," Briony wrote on Facebook.

"His mood is the lowest it's been in a very long time, which is probably the hardest part.

"There are no smiling photos to share right now. He is terrified, as am I.

"He's unable to read any of the texts, posts or emails that he receives, and he's often too tired to listen to me read them out.

"Hopefully we'll get a proper diagnosis very soon, and can get on with overcoming these latest obstacles."

Briony also wrote that her husband's weight is down to 74kgs, dropping up to 5kgs in only a few days since being admitted to hospital.

Lyle, from Shepparton in country Victoria, made an emotional comeback to the golf course during the 2013 Australian Masters before trying his luck at using a medical exemption to win his PGA Tour card back in 2015.

He then returned to Australia permanently to be with Briony and daughters Lusi and Jemma.

Australian Associated Press