He was released from prison in 1960 as part of an amnesty. He and his wife immigrated to the United Kingdom following the Warsaw Pact invasion of Czechoslovakia in 1968, which had crushed the Prague Spring. Krejci became a professor of sociology at Lancaster University. He taught in the university's Departments of French Studies, German Studies and Religious Studies from 1969 until 1983. Much of his research focused on history and the formation of civilization from a macrosociological approach. He returned to Czechoslovakia and the Czech Republic following the fall of communism to teach at Charles University in Prague and Palacký University, Olomouc, as well as the Diplomatic Academy of Vienna.