 The Australian Security Intelligence Organisation ( ASIO ) opened a file on Burhop in 1948 , believing him to be a secret member of the Communist Party , and an associate of Ian Milner , who was known through Venona intercepts to have passed secret documents to the Soviet Union . In July 1951 , the British government cancelled his passport when he accepted an offer to travel to the Soviet Union . A new passport was issued after he gave the Foreign Secretary a written assurance that he would not seek to travel to the Soviet Union or other Iron Curtain countries . 
