Pele has been moved to palliative end-of-life care at a hospital in Sao Paulo, according to reports in Brazil.
The 82-year-old, regarded as among the greatest footballers in history, had been receiving chemotherapy for bowel cancer.
However, Brazilian newspaper Folha de Sao Paulo reports he is no longer responding to treatment and he has been taken back into hospital where he is being spared invasive tests and treatment.
"Pray for the King," tweeted France forward Kylian Mbappe in response to the news. The Brazilian Football Confederation was unable to offer any immediate comment when contacted by Telegraph Sport. His manager and the Albert Einstein hospital, in São Paulo, are also yet to comment.
Speaking from Qatar, Harry Kane, England captain, said: "We send our best wishes to him and all his family. He is an inspiration among our game, incredible footballer and person. To hear those words back then was special for me. I am always learning and he was spot on with his advice. It is sad to hear that news and the whole England set-up wish him well."
Pele, a three-time World Cup winner, had been undergoing treatment since September 2021, and had surgery to remove a tumour from his colon before Christmas last year.
But it was more recently diagnosed that his cancer cells had spread to his intestine, lung and liver.
Pele's latest hospital admission was initially thought to be due to general swelling and reported heart failure. But in an Instagram post last week, his daughter, Kely Nascimento, said there was "no emergency". She wrote: "Lots of alarm in the media today concerning my dad's health. He is in the hospital regulating medication. There is no emergency or new dire prediction. I will be there for New Years and promise to post some pictures."
A post on Pele's Instagram followed on Friday in which he told his 10.6million followers: "Friends, I am at the hospital making my monthly visit. It's always nice to receive positive messages like this. Thanks to Qatar for this tribute, and to everyone who sends me good vibes!"
The hospital had reportedly stated on Friday that Pele had an "adequate response to an antibiotic treatment" and was in a stable condition "with general improvement in health status".
Pele has been lauded as the greatest footballer of all time, having burst on to the global scene as a 17-year-old at the 1958 World Cup, helping Brazil to the first of their record five successes.
Injury affected Pele's contribution to the 1962 and 1966 finals, but he led Brazil to a third triumph, this time in Mexico in 1970 as part of what is widely regarded as the greatest international team of all time.
He remains Brazil's leading scorer, with 77 goals - two ahead of Neymar.