Cuba has told the UN special rapporteur on torture he cannot visit the island, despite an invitation issued last year.
The UN investigator Manfred Novak expressed deep disappointment after being told he could not visit before his mandate ends in October.
His visit would have been the first by a UN expert to specifically monitor torture and cruel treatment in Cuba.
Cuban officials said it did not need an "objective assessment" of its situation.
"I regret that, in spite of its clear invitation, the government of Cuba has not allowed me to objectively assess the situation of torture and ill-treatment in the country," Mr Novak wrote.
Cuba invited him to visit in February 2009, but despite several attempts to propose mutually acceptable dates, no agreement was reached, he said.
The invitation had been seen as a major breakthrough, as the Cuban authorities have never previously allowed outside observers into the island's jails, the BBC's Michael Voss in Havana says.
Cuban diplomats in Geneva denied the invitation had been withdrawn, and said Cuba was still hoping to agree dates for a visit.
But they said there was no need for an independent assessment of the human rights situation in the country.
"There has not been one case of extra-judicial execution or of forced disappearance in Cuba," an official statement said.
"Few countries can boast of the results achieved in Cuba in the treatment of people in prison and their full reinsertion into society," it added.
Cuba's communist government has become increasingly sensitive to international criticism of its treatment of jailed dissidents since one prisoner, Orlando Zapata, died on 23 February after a long hunger strike.
Last week the government began moving some of the 200 jailed dissidents to prisons closer to their families, under an agreement reached with the Roman Catholic church.
The authorities deny there are political prisoners, calling them mercenaries in the pay of the US.