Media reports have said he was poisoned on November 1 with thallium, a chemical used in rat poisons and insecticides.
"I feel very bad. I've never felt like this before - like my life is hanging on the ropes," Litvinenko said from his hospital bed in London.
Alexander Goldfarb, a friend who helped Litvinenko defect to Britain, found him "more tired today, more exhausted" during a hospital visit on Monday.
He said that because Litvinenko's bone marrow was not functioning there was a risk that his heart or kidneys could fail.
British police have begun interviewing possible witnesses, including Litvinenko himself, examining his movements around the time of the apparent poisoning, and closed-circuit television footage.
Goldfarb and other friends of Litvinenko say that the FSB targeted the outspoken defector, who was granted political asylum in Britain in 2001.
"It's linked to Moscow and the FSB," Goldfarb said, adding that Litvinenko had written two books accusing the Russian secret services of criminal activites. "He was a very vocal critic of President Putin."
Thallium attacks the nervous system lungs, heart, liver and kidneys.
The colourless, odourless toxin results in hair loss, vomiting and diarrhoea. One gram can be enough to kill.