The shift came as Prime Minister Morgan Tsvangirai and ministers drawn from his MDC party boycotted a cabinet meeting led by President Robert Mugabe for the second time.
UN Special Rapporteur on torture and cruel, inhuman and degrading treatment Manfred Nowak was already in South Africa on his way to Zimbabwe for the week-long mission from Wednesday.
He called on the government in Harare to allow the visit to go ahead as planned despite the political turmoil.
"Upon his arrival in Johannesburg, on transit to Harare, the Special Rapporteur was informed that the mission had been postponed by the Government on 26 October 2009," the UN human rights office said in a statement.
UN officials declined to say if new dates had been offered and the statement also said the invitation had been withdrawn or cancelled.
It underlined the urgency of the fact-finding mission, highlighting allegations that MDC supporters had been arrested and intimidated in recent days.
"The Special Rapporteur therefore calls upon the government of Zimbabwe to receive him in Harare and allow the mission to go ahead as planned," the statement added.
Nowak had announced the invitation to check on conditions in Zimbabwe earlier this month, welcoming it as a sign that the government was willing to open dialogue on human rights and allow "unfettered access" to detention centres.
But Harare suddenly announced that it could not maintain the proposed dates citing a "previously unanticipated consultative process" with the Southern African Development Community (SADC).
An SADC team is heading to Harare to help resolve the political crisis.
Nowak welcomed the SADC initiative and all efforts to resolve the political crisis, and offered to be flexible during his mission.
"He fails to be convinced, however, that the Consultative process on Thursday, 29 October should be a valid reason to cancel his eight-day mission at such a late stage," the statement added.