Barcelona's French international defender Eric Abidal was scheduled to undergo a liver transplant Tuesday, a club spokesman said without confirming whether the operation had begun.
"I confirm that the operation was scheduled for today but I don't know whether it has started," the Barcelona spokesman told AFP.
"We have not received any information to the contrary. We are in the hands of the doctors, waiting to see what they will say."
Barcelona-based sports daily Sport said Abidal entered the operating block of BarnaClinic at 10 am (0800 GMT) and the surgery was expected to last between six and eight hours.
The 32-year-old defender was to receive a liver donated by his cousin Gerard, the paper reported on its online edition.
Barcelona announced March 15 that he would have a liver transplant "in the coming weeks", nearly a year after the player had a tumor removed from the same organ.
The club did not give any indication of how long he would be out of action but the operation puts him in doubt for the French team in the June 8-July 1 Euro 2012 finals in Poland and Ukraine.
Shortly after the club made the announcement, Barcelona coach Pep Guardiola said he was confident that Abidal would return to the pitch after getting a new liver.
"I am sure he will return. Not having him available to play is not what is important. He is irreplaceable for us. We don't have a substitute. He gives us an infinite range of possibilities," Guardiola said.
But the internationally-renowned head of Spain's organ transplant service, Rafael Matesanz, has warned that it would be "very difficult" for Abidal to play again after undergoing a transplant.
"It's clear that everything should go well and that he could play sport at a high level but football is a contact sport and one of the things that you have to avoid after a transplant is trauma and blows in that area," he told sports daily Marca last month.
"He has to take a series of precautions for the rest of his life, including not getting hit in that area. He will have medical treatment for life and will have to have regular check-ups."
Abidal made a rapid recovery after undergoing surgery to remove a tumor from his liver in March 2011 and drew loud cheers as he made a symbolic return on May 3 in the final minutes of Barca's Champions League semi-final second leg tie with Real Madrid.
"The Camp Nou's ovation in the semi finals of the Champions League was one of the most amazing things I've ever seen. He's a very good person and very much loved by the squad," Guardiola said last month.
"If people love him so much it's because he has earned it. I'm sure that that will give him a lot of strength to get through this."
Abidal has been a regular this season for the Catalan side, starting in 20 first division matches.
But he has not played since picking up a hip injury playing for France in their friendly win over Germany at the start of March.
His contract had been due to expire at the end of the current season but in January he agreed a new one-year deal with Barcelona which ties him to the club until 2013 with options to extend it by two more years.