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Boxing legend Muhammad Ali is thought to be in a serious condition and placed on a life-support machine after he was hospitalised for a “respiratory issue”.

The 74-year-old was taken to hospital in Phoenix, Arizona, on Thursday to be treated as a precaution, his spokesman said.

He is said to be suffering respiratory issues that are reportedly complicated by the Parkinson’s disease that was diagnosed in the 1980s.

But two people close to the former heavyweight champion have said that Ali’s condition is concerning to his family members.

A source told the Mirror: “Like in the ring, Ali is a fighter on the ward. Doctors are working to regulate his breathing put it is being hampered by his Parkinson’s.

“His children are all extremely concerned and dropped everything to be with him. They fear the worst.

“Ali is everything to them and there are worried his problems are worse than first feared.”

The sources have reportedly declined to be identified because they were not speaking on behalf of Ali’s family.

Muhammad Ali - In pictures 35 show all Muhammad Ali - In pictures 1/35 PARIS - JANUARY 1: A portrait of former World boxing heavyweight champion Muhammad Ali (born Cassius Clay) in 1960 in Paris, France. (Photo by AFP/Getty Images) AFP/Getty Images 2/35 The winners of the 1960 Olympic medals for light heavyweight boxing on the winners' podium at Rome: Cassius Clay (now Muhammad Ali) (C), gold; Zbigniew Pietrzykowski of Poland (R), silver; and Giulio Saraudi (Italy) and Anthony Madigan (Australia), joint bronze Central Press/Getty Images 3/35 27th May 1963: American Heavyweight boxer Cassius Clay (now Muhammad Ali) lying on his hotel bed in London. He holds up five fingers in a prediction of how many rounds it will take him to knock out British boxer Henry Cooper Len Trievnor/Express/Getty Images 4/35 Muhammad Ali (born Cassius Clay) leaves his London hotel for early morning training in preparation for his rematch with Henry Cooper, accompanied by two police officers 5/35 Muhammad Ali (born Cassius Clay) suits up as a City gentleman in 1963 ahead of his stay in London Hulton Archive/Getty Images 6/35 Cassius Clay (now Muhammad Ali) being knocked down by Henry Cooper in 1963 at the Empire Stadium, Wembley in London 7/35 British Heavyweight champion Henry Cooper (1934 - 2011, left) with American boxer Cassius Clay (now Muhammad Ali) after their non-title fight at Wembley Stadium, London, 18th June 1963. Clay won the match by a technical knockout after Cooper sustained a cut under his left eye Central Press/Hulton Archive/Getty Images 8/35 Muhammad Ali (born Cassius Clay) jumps over a fence in London's Hyde Park following his win over Henry Cooper Daily Mail 9/35 The Beatles take a fake punch From Muhammad Ali (born Cassius Clay) while visiting the heavyweight contender's training camp in Miami Beach in 1964 Rex 10/35 Elijah Muhammad giving speech with Cassius Clay (now Muhammad Ali) present in 1964 11/35 American boxer Cassius Clay (now Muhammad Ali) with boxing manager Chris Dundee around the time of his World Heavyweight Boxing Championship fight against fellow American Sonny Liston at Miami Beach, Florida. Ali went on to win the match, securing his first world heavyweight title Harry Benson/Getty Images 12/35 25 May 1965: Cassius Clay (now Muhammad Ali) stands over Sonny Liston after a first round knockout to claim the heavyweight champion title in Lewiston, Maine Allsport/Hulton Archive/Getty Images 13/35 Cassius Clay (now Muhammad Ali) being restrained by his seconds as he rushes across the ring after his world heavyweight title fight victory over Sonny Liston, yelling to the ringside reporters 'eat your words' Central Press/Getty Images 14/35 Muhammad Ali in training in August 1966 for his upcoming fight against Brian London. London lost the fight, after being knocked out in the third round R. McPhedran/Express/Getty Images 15/35 Cassius Clay (now Muhammad Ali) spars with challenger Floyd Patterson during the World Heavyweight Championship fight on 25 November 1965. Clay won the fight with a technical knockout in the 12th round Allsport/Getty Images 16/35 6 August 1966: Cassius Clay (Muhammad Ali) lands a right to the head of Brian London during their World Heavyweight Title fight at Earls Court. Ali won by a knockout in the third round Allsport/Hulton Archive/Getty Images 17/35 March 1971: In a title fight at Madison Square Gardens, New York, Muhammad Ali goes down in the 15th round to a left hook from world heavyweight champion Joe Frazier who kept the title with an unanimous points win Keystone/Getty Images 18/35 Muhammad Ali (born Cassius Clay) winces as Ken Norton hits him with a left to the head during their re-match at the Forum in Inglewood, California in 1973 AP 19/35 The American boxer Muhammad Ali (born Cassius Clay), who first became world heavyweight champion in 1964 read the London Evening Standard in 1974 Tim Graham/Getty Images 20/35 "The Match of the Century " between the former world heavyweight boxing champion Muhammad Ali (right) and his compatriot and titleholder George Foreman (left). Ali won and got back his title on 30 October 1974 in Kinshasa, Congo AFP/Getty Images 21/35 9th May 1966: Cassius Clay (now Muhammad Ali) at his West End Hotel during his stay in London to fight Henry Cooper B. Potter/Express/Getty Images 22/35 Muhammad Ali (born Cassius Clay) fighting Joe Bugner in 1973 Daily Mail 23/35 Referee Zack Clayton counts out George Foreman as Muhammad Ali looks on in the eighth round of their title bout in Kinshasa, Zaire on 30 October 1974 Jim Boudier/AP 24/35 Spray flies from the head of challenger Joe Frazier as heavyweight champion Muhammad Ali (born Cassius Clay) connects with a right in the ninth round of their title fight in Manila, Philippines on October 1, 1975. Ali won the fight on a decision to retain the title Mitsunori Chigita/AP 25/35 Pele is shown embracing Muhammad Ali during a ceremony honoring the Brazilian star of the New York Cosmos at Giants Stadium, East Rutherford, New Jersey on 1 October 1977 AP 26/35 World heavyweight boxing champion Muhammad Ali (left) during the match he won against his compatriot Leon Spinks in Moscow on 16 September 1978 AFP/Getty Images 27/35 Michael Parkinson with Muhammad Ali (born Cassius Clay) on the BBC chat show in 1981 BBC 28/35 19 December 1978: Heavyweight boxer Muhammad Ali with his daughters Laila (9 months) and Hanna (2 years 5 months) at Grosvenor House Frank Tewkesbury/Evening Standard 29/35 Former heavyweight boxing champion and 1960 OlympIc gold medallist Muhammad Ali lights the flame at the Olympic Stadium in Atlanta during the opening ceremony for the 1996 Olympics Omar Torress/AFP/Getty Images 30/35 11 December 1999: Boxing legends Lennox Lewis, Harry Carpenter and Muhammad Ali pose for the cameras at the BBC Sports Personality of the Year Awards ceremony held at the BBC Television Centre, in London Craig Prentis/Allsport/Getty Images 31/35 Muhammad Ali during his Birthday Party at the London Hilton in 2001 John Gichigi/Allsport/Getty Images 32/35 Muhammad Ali (born Cassius Clay) lights the first Olympic torch for the Salt Lake 2002 Olympic Winter Games at Centennial Olympic Park in Atlanta on 4 December 2001 Curtis Compton/EPA 33/35 US former world heavyweight boxing champion Muhammad Ali (C) poses with two of his daughters, Hana (left) and May May (right), after a star was unveiled for him on the Hollywood Walk of Fame, in Los Angeles on 11 January 2002 Lucy Nicholson/AFP/Getty Images 34/35 Boxer Laila Ali poses with her father, former heavyweight champion Muhammad Ali, after she defeated Suzy Taylor in two rounds at the Aladdin Casino in Las Vegas, Nevada on 17 August 2002 Scott Halleran/Getty Images 35/35 U.S. President George W. Bush presents former heavyweight boxing champion Muhammad Ali with the Medal of Freedom during a ceremony during a ceremony in the East Romm of the White House November 9, 2005 in Washington, DC. Bush presented 14 Medals of Freedom to recipients including retiring Federal Reserve Chairman Alan Greenspan and Aretha Franklin during the ceremony on 09 November 2005 Mark Wilson/Getty Images 1/35 PARIS - JANUARY 1: A portrait of former World boxing heavyweight champion Muhammad Ali (born Cassius Clay) in 1960 in Paris, France. (Photo by AFP/Getty Images) AFP/Getty Images 2/35 The winners of the 1960 Olympic medals for light heavyweight boxing on the winners' podium at Rome: Cassius Clay (now Muhammad Ali) (C), gold; Zbigniew Pietrzykowski of Poland (R), silver; and Giulio Saraudi (Italy) and Anthony Madigan (Australia), joint bronze Central Press/Getty Images 3/35 27th May 1963: American Heavyweight boxer Cassius Clay (now Muhammad Ali) lying on his hotel bed in London. He holds up five fingers in a prediction of how many rounds it will take him to knock out British boxer Henry Cooper Len Trievnor/Express/Getty Images 4/35 Muhammad Ali (born Cassius Clay) leaves his London hotel for early morning training in preparation for his rematch with Henry Cooper, accompanied by two police officers 5/35 Muhammad Ali (born Cassius Clay) suits up as a City gentleman in 1963 ahead of his stay in London Hulton Archive/Getty Images 6/35 Cassius Clay (now Muhammad Ali) being knocked down by Henry Cooper in 1963 at the Empire Stadium, Wembley in London 7/35 British Heavyweight champion Henry Cooper (1934 - 2011, left) with American boxer Cassius Clay (now Muhammad Ali) after their non-title fight at Wembley Stadium, London, 18th June 1963. Clay won the match by a technical knockout after Cooper sustained a cut under his left eye Central Press/Hulton Archive/Getty Images 8/35 Muhammad Ali (born Cassius Clay) jumps over a fence in London's Hyde Park following his win over Henry Cooper Daily Mail 9/35 The Beatles take a fake punch From Muhammad Ali (born Cassius Clay) while visiting the heavyweight contender's training camp in Miami Beach in 1964 Rex 10/35 Elijah Muhammad giving speech with Cassius Clay (now Muhammad Ali) present in 1964 11/35 American boxer Cassius Clay (now Muhammad Ali) with boxing manager Chris Dundee around the time of his World Heavyweight Boxing Championship fight against fellow American Sonny Liston at Miami Beach, Florida. Ali went on to win the match, securing his first world heavyweight title Harry Benson/Getty Images 12/35 25 May 1965: Cassius Clay (now Muhammad Ali) stands over Sonny Liston after a first round knockout to claim the heavyweight champion title in Lewiston, Maine Allsport/Hulton Archive/Getty Images 13/35 Cassius Clay (now Muhammad Ali) being restrained by his seconds as he rushes across the ring after his world heavyweight title fight victory over Sonny Liston, yelling to the ringside reporters 'eat your words' Central Press/Getty Images 14/35 Muhammad Ali in training in August 1966 for his upcoming fight against Brian London. London lost the fight, after being knocked out in the third round R. McPhedran/Express/Getty Images 15/35 Cassius Clay (now Muhammad Ali) spars with challenger Floyd Patterson during the World Heavyweight Championship fight on 25 November 1965. Clay won the fight with a technical knockout in the 12th round Allsport/Getty Images 16/35 6 August 1966: Cassius Clay (Muhammad Ali) lands a right to the head of Brian London during their World Heavyweight Title fight at Earls Court. Ali won by a knockout in the third round Allsport/Hulton Archive/Getty Images 17/35 March 1971: In a title fight at Madison Square Gardens, New York, Muhammad Ali goes down in the 15th round to a left hook from world heavyweight champion Joe Frazier who kept the title with an unanimous points win Keystone/Getty Images 18/35 Muhammad Ali (born Cassius Clay) winces as Ken Norton hits him with a left to the head during their re-match at the Forum in Inglewood, California in 1973 AP 19/35 The American boxer Muhammad Ali (born Cassius Clay), who first became world heavyweight champion in 1964 read the London Evening Standard in 1974 Tim Graham/Getty Images 20/35 "The Match of the Century " between the former world heavyweight boxing champion Muhammad Ali (right) and his compatriot and titleholder George Foreman (left). Ali won and got back his title on 30 October 1974 in Kinshasa, Congo AFP/Getty Images 21/35 9th May 1966: Cassius Clay (now Muhammad Ali) at his West End Hotel during his stay in London to fight Henry Cooper B. Potter/Express/Getty Images 22/35 Muhammad Ali (born Cassius Clay) fighting Joe Bugner in 1973 Daily Mail 23/35 Referee Zack Clayton counts out George Foreman as Muhammad Ali looks on in the eighth round of their title bout in Kinshasa, Zaire on 30 October 1974 Jim Boudier/AP 24/35 Spray flies from the head of challenger Joe Frazier as heavyweight champion Muhammad Ali (born Cassius Clay) connects with a right in the ninth round of their title fight in Manila, Philippines on October 1, 1975. Ali won the fight on a decision to retain the title Mitsunori Chigita/AP 25/35 Pele is shown embracing Muhammad Ali during a ceremony honoring the Brazilian star of the New York Cosmos at Giants Stadium, East Rutherford, New Jersey on 1 October 1977 AP 26/35 World heavyweight boxing champion Muhammad Ali (left) during the match he won against his compatriot Leon Spinks in Moscow on 16 September 1978 AFP/Getty Images 27/35 Michael Parkinson with Muhammad Ali (born Cassius Clay) on the BBC chat show in 1981 BBC 28/35 19 December 1978: Heavyweight boxer Muhammad Ali with his daughters Laila (9 months) and Hanna (2 years 5 months) at Grosvenor House Frank Tewkesbury/Evening Standard 29/35 Former heavyweight boxing champion and 1960 OlympIc gold medallist Muhammad Ali lights the flame at the Olympic Stadium in Atlanta during the opening ceremony for the 1996 Olympics Omar Torress/AFP/Getty Images 30/35 11 December 1999: Boxing legends Lennox Lewis, Harry Carpenter and Muhammad Ali pose for the cameras at the BBC Sports Personality of the Year Awards ceremony held at the BBC Television Centre, in London Craig Prentis/Allsport/Getty Images 31/35 Muhammad Ali during his Birthday Party at the London Hilton in 2001 John Gichigi/Allsport/Getty Images 32/35 Muhammad Ali (born Cassius Clay) lights the first Olympic torch for the Salt Lake 2002 Olympic Winter Games at Centennial Olympic Park in Atlanta on 4 December 2001 Curtis Compton/EPA 33/35 US former world heavyweight boxing champion Muhammad Ali (C) poses with two of his daughters, Hana (left) and May May (right), after a star was unveiled for him on the Hollywood Walk of Fame, in Los Angeles on 11 January 2002 Lucy Nicholson/AFP/Getty Images 34/35 Boxer Laila Ali poses with her father, former heavyweight champion Muhammad Ali, after she defeated Suzy Taylor in two rounds at the Aladdin Casino in Las Vegas, Nevada on 17 August 2002 Scott Halleran/Getty Images 35/35 U.S. President George W. Bush presents former heavyweight boxing champion Muhammad Ali with the Medal of Freedom during a ceremony during a ceremony in the East Romm of the White House November 9, 2005 in Washington, DC. Bush presented 14 Medals of Freedom to recipients including retiring Federal Reserve Chairman Alan Greenspan and Aretha Franklin during the ceremony on 09 November 2005 Mark Wilson/Getty Images

According to reports, Ali’s family have been told to prepare for the worst.

What is Parkinson's disease? Parkinson's is a neurologic disease that robs people of control over their movements. It typically starts with tremors, and is characterised by slow movement, a shuffling gait, stiff limbs, balance problems and slurred speech. Who gets it? An estimated 4 million to 5 million people worldwide are living with the disease, according to the National Parkinson Foundation. It usually appears after age 60, although sometimes it can develop before age 40. What causes it? The exact cause isn't known but Parkinson's develops when cells that produce one of the brain's chemical messengers, called dopamine, begin to deteriorate and die. Dopamine transports signals to parts of the brain that control movement. Is there a cure? There is no cure but there are a range of treatments, from medications that affect dopamine levels to a surgically implanted tremor-blocking device. Patients also can benefit from physical and occupational therapy. What is the prognosis?Symptoms worsen over time, usually slowly. The severity of symptoms, and how quickly they progress, varies widely between patients. While Parkinson's itself isn't considered fatal, people can die from complications of the disease.

Bob Gunnell, Ali’s spokesman, said in a statement on Thursday that the former heavyweight champion was in a “fair” condition and a brief hospital stay was expected.

Mr Gunnell said in an email Friday there was no update on Ali's condition.

Dr Abraham Lieberman, the boxer’s long term Parkinson’s doctor, declined to comment any further on Ali’s condition when contacted by the Associated Press.

He said: “I can’t really say much more than what’s in the papers.”

The boxing great has been hospitalised several times in recent years - most recently in early 2015 when he was treated for a severe urinary tract infection, initially diagnosed as pneumonia.

Ali’s last public appearance was on April 9 for the annual Celebrity Fight Night dinner, which raises funds for treatment of Parkinson’s.