Muhammad Ali remains in hospital amid reports of 'grave' concerns over his health

The former world heavyweight champion is being treated for a respiratory problem

Muhammad Ali has suffered from Parkinson's disease for more than 30 years (Rex)

Legendary boxer Muhammad Ali remains in hospital, with concern said to be growing over his condition.

The 74-year-old was rushed to hospital in Phoneix, Arizona on Thursday and is being treated for a respiratory issue.

The Associated Press reported that Ali's stay in hospital may involve more serious issues than in previous vists, while a sorce told Reuters that the situation was "extraordinarily grave".

One US gossip website, Radar Online, was reporting on Friday night that Ali was on life support and "barely breathing", according to sources, and that his family were "prepared for the worst"

A number of family members had reportedly gathered around Ali's bedside on Friday.

Laila Ali, herself a former boxing champion, posted a picture Friday afternoon on Facebook of her father holding her daughter when she was an infant.

“I love this photo of my father and my daughter Sydney when she was a baby!” she wrote. “Thanks for all the love and well wishes. I feel your love and appreciate it!!”

There was no official update about Ali's condition on Friday, Bob Gunnell, the family spokesman, had said that Ali was in a fair condition and was only expected to be in a hospital for a brief period. .

Muhammad Ali won the heavyweight championship title three times (Getty Images) (Getty)

The former heavyweight champion has been in and out of hospital several times in the past few years and was treated for a severe urinary tract infection in early 2015.

He has suffered from Parkinson's for three decades, most famously trembling badly while lighting the Olympic torch in 1996 in Atlanta. Despite the disease he kept up a busy appearance schedule until recently, though he has not spoken in public for years.

Nicknamed “The Greatest” Ali retired from boxing in 1981 with a 56-5 record. Ali's diagnosis of Parkinson's came about three years after he retired.

Dr Abraham Lieberman of the Barrow Neurological Insitute in Phoneix, who has treated Ali for Parkinson's disease for several years, declined to comment on his patient's condition saying he couldn't say "much more than what's in the papers".

Additional reporting by AP