The UN torture investigator said Bahrain had effectively cancelled a trip he had planned to the Gulf Arab state, where unrest led by majority Shi'ites against the Sunni ruling family has simmered for over two years.
Juan Mendez, the UN Special Rapporteur on Torture, said in a statement it was the second time a scheduled visit had been postponed at short notice. His trip had been set for May 8-15.
"It is effectively a cancellation as no alternative dates were proposed, nor is there a future road map to discuss," he said on Wednesday. "Let me be clear, this was a unilateral decision by the authorities."
Bahrain's state news agency said on Monday the authorities had asked Mendez to delay his trip, without giving a reason. Mendez then put it off until further notice, the agency said.
Bahrain, which hosts the US Fifth Fleet, crushed Shi'ite-led pro-democracy demonstrations that began in February 2011. At least 35 people were killed. Lower-level unrest continues.
Under criticism from human rights groups, the government invited a commission led by Cherif Bassiouni, a respected UN human rights lawyer to examine its handling of the trouble. Its report in November 2011 said the authorities had used widespread and excessive force, including torture to extract confessions.
The Bahrain government says it has taken steps to address the brutality of security forces by dismissing those responsible and introducing cameras at police stations to monitor abuses.
Mendez had been due to meet government officials, members of civil society groups and political party members, and to visit places of detention.
Amnesty International said Bahrain's decision showed it was clearly "not serious" about implementing human rights reforms.
In its annual report on human rights, the US State Department said this month Bahrain had failed to implement the most important recommendations detailed in the Bassiouni report.
The State Department report said Bahrain's most serious human rights problems included the arrest and detention of protesters on vague charges, in some cases leading to torture while being detained, and a lack of due process in trials of activists, medical personnel, teachers and students.
The report also said some protesters had engaged in "lethal acts of violence against security forces".
Bahraini Information Minister Samira Rajab rejected the report's findings, saying it lacked objectivity.
"The report includes texts which are totally far from the truth, adopting a manner that fuels terror and terrorists targeting Bahrain's national security," the Bahraini news agency quoted Rajab as saying on Wednesday.