Former Libyan intelligence officer Abdelbaset al-Megrahi's health is said to have deteriorated quickly.
Al-Megrahi's son, Khaled, said his father was taken to hospital for the second time in two days.
"My dad's health is very bad and has been worsening," Khaled said at the family home in Tripoli. "He is on his last breath. I don't think he can make it this time."
Al-Megrahi's brother Abdulhakim earlier told Reuters: "His health began to deteriorate quickly and we were worried about him, so took him immediately to the hospital where he is receiving a blood transfusion."
Al-Megrahi is the only man convicted of the bombing, which killed 270 people when Pan Am Flight 103 blew up over Lockerbie four days before Christmas in 1988.
He was sentenced to life in prison in 2001, but was released on compassionate grounds in August 2009 after being diagnosed with terminal cancer.
At the time, doctors estimated he had around three months to live.
Since his return to Libya, al-Megrahi has rarely appeared in public. His family has on several occasions said that he is near death, in what was seen as an attempt to justify his release.
