Legislation outlawing apostasy - the act of changing religious affiliation - and insults against religious figures could be used to violate the rights of minorities, Heiner Bielefeld said in a report to the U.N. Human Rights Council.
The comments from the United Nations' special rapporteur on freedom of religion or belief came amid heightened focus on faith-based laws in countries like Saudi Arabiaand Pakistan, where blasphemy carries the death penalty.
"States should repeal any criminal law provisions that penalize apostasy, blasphemy and proselytism, as they may prevent persons belonging to religious or belief minorities from fully enjoying their freedom of religion or belief," he said in the report.
Rights campaigners say the blasphemy law in Pakistan is widely used against religious minorities, including Christians, Ahmadis and more recently Shiah Muslims, usually on flimsy pretexts.
The posting of an amateurish U.S.-made video mocking the Prophet Mohammad, and the publication of caricatures of him in France last year led to violent protests and renewed calls from the Muslim world for a global law against blasphemy.
Speaking on the fringes of the rights council on Wednesday, Bielefeld said criminalizing concepts like blasphemy was dangerous for free speech because there could be no common definition of what it was.