He told The Sun: ‘He’s in a bad way. He’s very sick.
‘It could be months, it could be days. I don’t know if he’ll last the summer. He’s in God’s hands.’
Ali was diagnosed with Parkinson’s disease in 1984 but remained active for many years and made a moving appearance at the Atlanta 1996 Olympic Games where he lit the flame.
However, he was seen looking particularly thin and frail at the London 2012 Olympic opening ceremony where he was helped across the stage by wife Lonnie.
Rahman has accused his sister-in-law, who has been married to Ali for 26 years, of refusing to let the family visit the ailing champion.
Rahman, who was also a professional boxer, said he last saw his brother in July at the Sports for Peace Gala in London and is only allowed to speak to him over the phone.