Preparations for the conference have been beset by problems
US Secretary of State Colin Powell will not attend the World Conference against Racism due to begin in Durban, South Africa, on Friday, the State Department has confirmed.
A department official said the US was still debating whether to send any representation at all to the conference.
Just hours earlier, the United Nations human rights chief, Mary Robinson, said delegates had agreed that moves to equate Zionism with racism would be off the agenda.
'Offensive' language
The absence of Mr Powell will be regarded as a significant blow by the organisers, not least UN Secretary-General Kofi Annan.
The two men have been in regular contact over the last few days as Mr Annan tried to persuade Mr Powell that US concerns about the wording of preparatory documents were being heeded.
But the effort at persuasion failed - a State Department spokesman cited offensive language about Israel and a singling out of Israel as the reasons for Mr Powell's decision not to travel to Durban.
"The exact nature of our representation, if any, is still being considered," he said.
The Americans have focused most of their discontent on wording in preparatory documents which equated Zionism with racism.
Slave reparations
There was also unhappiness with apparent support for reparations for slavery, but it was the condemnation of Israel which seems to have been decisive.
Human rights groups and non-governmental agencies around the world, while not surprised by the US announcement, are likely to be highly critical.
Michael McClintock of Human Rights Watch said it reflected a wider US trend toward unilateralism.
However, the absence of Mr Powell from Durban is sure to be welcomed by Jewish groups, who have loudly condemned what they describe as efforts to impose an anti-Israel agenda on the Durban conference.