Barcelona coach Tito Vilanova has suffered a relapse on the cancer in his saliva glands, Spanish daily El Pais reported on Wednesday citing sources at the club.
The 44-year-old Vilanova, who underwent surgery on a tumour in his glands in November last year, stepped up from his role as assistant to replace Pep Guardiola as first-team coach for this season.
The La Liga leaders put out a statement cancelling a Christmas event planned with local media and president Sandro Rosell in the morning, saying the reasons would be given later.
"Very sorry to hear the news," Barca midfielder Xavi said on his official Twitter account, as messages of support flooded in.
Former tennis World No 1 Rafa Nadal said: "All my strength and support for Tito Vilanova! We are all with you to overcome this next hurdle."
Vilanova was out of action for around three weeks after surgery last year, but returned to the dugout to help Guardiola for the last six months of the 2011-12 campaign.
The quietly-spoken Catalan said the experience had left him with a different perspective on life, at his presentation in June.
"I had an important operation a few months ago," he said.
"I spoke with the medics and my family about the job, but they all said I was perfectly okay and there was no problem.
"Having overcome this operation it gives me a different view on life. Perhaps, after what has happened, training Barca will be like kids play."
Vilanova has guided the team to the top of La Liga with 15 wins in 16 games, nine points clear of second-placed Atletico Madrid, and into the last 16 of the Champions League as group winners.
Barca have one game left, away at Valladolid in La Liga on Saturday, before the two-week winter break.