Javier Solana, the US foreign policy chief who had been scheduled to meet Ali Larijani, Iran's chief nuclear negotiator, said he was surprised by the decision on Wednesday.
"I had made it clear to the Iranians and to Dr Larijani that we want to proceed rapidly to examine together the ideas I put to him early last month," he said.
He said the meeting would be rescheduled for Thursday, with a second meeting on July 11, the day an informal deadline is set by Western diplomats for Iran to suspend uranium enrichment and agree to talks on its nuclear programme expires.
Iran did not give a reason for the postponement, but EU diplomatic sources said the Islamic republic was angered by
a visit to the European Parliament in Strasbourg on Wednesday by the leader of an Iranian opposition group.
Maryam Rajavi from the National Council of Resistance of Iran, which Iran deems a terrorist group, was invited to the parliament by several EU politicians.
Rajavi told the parliament on Wednesday that the group had "reliable information" that Iran had no intention of halting uranium enrichment.
Iran has been under Western pressure to say whether the package is acceptable by next week, but on Tuesday Larijani said that Tehran could wait until August to respond.
Iran has said that an offer made on June 6 by Germany and the five permanent United Nations Security Council members - the US, Britain, France, Russia and China - contained ambiguities.
The powers offered a state-of-the-art nuclear reactor with a guaranteed fuel supply, economic benefits and support for the idea of a regional security framework.
In return, Iran would suspend all enrichment-related activities and accept wider inspections by the UN's nuclear watchdog.
Iran maintains that it is developing nuclear technology for peaceful purposes, but some in the West fear it is trying to make nuclear weapons.