The revelations have been disclosed by the newspaper Expressen after it obtained documents from Swedish intelligence agency Säpo on Guillou's relations with the KGB.
The documents centre around Russian agent Jevgenij Ivanovitj Gergel, the KGB's man in Stockholm at the end of the 1960s.
A witness statement from one of Guillou's journalist colleagues at the time raised the alarm over relations between the two. It also refers to an assignment to steal an internal telephone directory from the American Embassy in Stockholm.
Guillou confirmed that he first met Gergel at a reception held at the Soviet Embassy in Stockholm in 1967.
”We never did anything other than talk politics,” he told the newspaper.
Guillou adds that his connection never led to any journalistic revelations and he denies spying for the Soviets.
He concedes, however, that he undertook paid assignments but claims the purpose was of a professional nature, to investigate how the KGB was working in Sweden at the time.
”It was just a few non-events and it is not a crime to meet foreign intelligence services,” he added.
Guillou had contact with the KGB until 1972 when he began publishing articles that revealed the existence of Informationsbyrån, a secret Swedish military intelligence agency that spied on Swedish citizens for political purposes. He was later jailed for espionage.
Säpo's investigation of Guillou's KGB relations never led to any indictments writes Expressen.