Bahrain’s decision “does not enhance transparency with regard to the situation in the country, nor demonstrate a commitment to redress impunity regarding any violations,” said the expert, Juan E. Mendez, in a statement released in Geneva. Mr. Mendez is based there as the United Nations’s special rapporteur on torture.

The cancellation follows a week of clashes between the police and opposition demonstrators in Bahrain, mostly in villages outside Manama, the capital. They were timed to coincide with a Formula One auto race in Manama, which attracts international media attention. The race took place on Sunday without incident, but the protests signaled a simmering challenge to the ruling Al Khalifa family.

Mr. Mendez had been scheduled to meet a number of key government ministers and officials during a visit that had been discussed since September 2011. An independent commission of inquiry reported late that year that some detainees in Bahrain had been tortured to death and others subjected to physical and psychological abuse to extract confessions or as punishment. The commission and the U.N. Human Rights Council recommended a number of reforms; Mr. Mendez said in a telephone interview that his visit would have given him an opportunity to see how much the Bahrain government had done to implement them.

He was originally scheduled to go to Bahrain in February 2012, but the Bahraini authorities canceled the visit on short notice, saying that there had not yet been enough progress on the reforms. Similarly, the letter delivered to Mr. Mendez this week canceling next month’s visit said that talks with the opposition in Bahrain had not progressed as fast as expected and that the visit could damage their chances of success.

A government statement carried by the Bahrain News Agency said that Mr. Mendez had postponed the trip. “Let me be clear,” Mr. Mendez said in his statement, “this was a unilateral decision by the authorities.”