Treholt, 63, is on a respirator and doctors believe he is suffering from blood poisoning, Harald Stabell told The Associated Press. They will try to take him off the respirator Thursday, Stabell said.
Treholt served eight years of a 20-year prison sentence for giving NATO secrets to the Soviet Union and Iraq.
Treholt, who has always denied spying, was admitted a week ago to the hospital in the port city of Limassol, Stabell said.
Stabell said Wednesday he was in Cyprus with a view to taking up Treholt's case again in Norway.
Treholt was a Norwegian Foreign Ministry press officer when he was arrested in January 1984 at Oslo's Fornebu Airport. Prosecutors said he was on his way to a meeting with a Soviet KGB official in Vienna.
The prosecutors also said Treholt began contact with diplomats he knew were KGB agents as early as 1967.
Treholt, the son of a former cabinet minister, was convicted in June 1985 of passing military secrets during the 1970s and 1980s, and was sentenced to the maximum 20 years.
He was released from prison and pardoned in 1992, shortly after the collapse of the Soviet Union brought the end of the Cold War.
He emigrated to Moscow after his release, and later moved to Cyprus, where he has been working as a writer and businessman.