Foreign Minister Franco Frattini said Italy was prevented from joining its partners in the Iran mission because of its role as the current president of the European Union.
"Their initiative was followed and supported by the presidency," Frattini told La Repubblica newspaper. "But it is true that we were penalised somewhat as the European presidency."
Iran bowed to international pressure on Tuesday and agreed to tougher inspections of nuclear sites and to suspend uranium enrichment programmes after talks in Tehran with the foreign ministers of Europe's "big three" -- Britain, France and Germany.
"If Italy claimed its ranking as a big nation during its EU presidency it would lose its credibility among the 'smalls' and that would be a serious mistake," Frattini added.
He also denied there was an elite club of the most powerful nations within the European Union, which is set to expand in May next year from 15 to 25 member states.
"Such a club should not exist and will not," Frattini said.
However, differences between the more powerful EU nations and the smaller states and incoming members have been highlighted as the Union tries to hammer out a new constitution.