The Bahraini government has postponed indefinitely a visit by Juan Mendez the UN special rapporteur on torture.
This comes just days after the release of a US State Department report on human rights in Bahrain which spoke of "significant" violations including torture in detention.
The Gulf kingdom has been wracked by civil unrest for two years.
The violence has left at least 50 people dead.
An independent enquiry established by King Hamad al-Khalifa in 2011 found that there had been numerous abuses. The king accepted the report and promised accountability and reform.
But human rights organisations in Bahrain and outside the country say that the promised reforms are happening either too slowly or not at all. And they allege that human rights abuses are continuing.
The US State Department Bahrain 2012 Human Rights Report spoke of "serious human rights problems," including "citizens' inability to change their government peacefully; arrest and detention of protesters on vague charges, in some cases leading to their torture in detention; and lack of due process in trials of political and human rights activists".
Mr Mendez had originally intended to visit Bahrain in 2012 but that too was called off. Pointing to this second postponement Brian Dooley of US based Human Rights First called the decision "a huge blow to the credibility of Bahrain's reform process".
"It seems like the Bahrain regime is frightened of what more international scrutiny might reveal. It's very telling that they've shut Mendez out again," said Mr Dooley.
A spokesperson in the British Foreign and Commonwealth Office (FCO) described the decision as "disappointing" and noted that Alistair Burt, the Parliamentary Under Secretary of State had raised the issue with the Bahraini government "stressing the importance we and the international community place on the visit".
The FCO said: "We hope that a new date for this visit can be found soon."