In a sharp U-turn, Ayatollah Ahmad Janati, a key aide of supreme leader Ayatollah Ali Khamenei, said the regime had acted with wisdom in accepting the demands of the International Atomic Energy Agency (IAEA) before the UN watchdog's deadline ran out Friday.
"This decision is that of the whole regime," Janati told worshippers at the main weekly prayers in Tehran.
"Incontestably, those who took this decision took into account the country's interests and are familiar with all the sensitivities of the dossier," said Janati, who just weeks ago demanded that Iran withdraw from the nuclear Non-Proliferation Treaty altogether rather than accept demands to sign its additional protocol.
In a reference to some 1,500 hardliners demonstrating outside the mosque against the regime's climbdown, Janati called on the faithful to avoid "adopting positions on matters you do not fully understand."
Janati heads the Guardians' Council, a constitutional watchdog body that is one of the bastions of conservative power here, and his views carry weight across Iran's religious right.
As they have done every Friday in recent weeks, the hardline protestors chanted: "Death to America, death to Britain," and denounced the regime's concessions to the international community as a "humiliation".
Janati attached a single caveat to his comments, warning the European brokers of Iran's climbdown that they must keep their side of the bargain.
"If the European side does not keep its word, all of Iran's commitments will be null and void," he warned.
In return for Iranian pledges to suspend uranium enrichment and sign up to the snap inspections of nuclear sites demanded by the additional protocol, Britain, France and Germany agreed to provide technical assistance for civil energy production, understood to include supplies of fuel.
An official of Iran's reformist-led government earlier rejected suggestions the regime might renege on its commitments to the international community.
"When we make an undertaking at the international level, we respect it," the official said, asking not be identified.
"We have handed over a full, clear and detailed report (on our nuclear activities). At the moment our cooperation (with the IAEA) goes beyond that required by the additional protocol.
"Once we have actually signed it, we will work within the framework laid down by the protocol."
Ahead of Friday's deadline set by the IAEA last month, Iran made a string of concessions to fall into line with the watchdog's demands.
Last week, it handed over a massive report on its nuclear programme, which IAEA director general Mohamed ElBaradei said Thursday "at first glance ... looks comprehensive."
During the unprecedented joint visit by the British, French and German foreign ministers earlier this month, Iran also fell in line with the nuclear watchdog's other demands.
Iran's archfoe, the United States, has since expressed some scepticism about the Islamic regime's readiness to deliver on its undertakings.
"(The Iranians) need to follow through on what they've committed to do and meet their international obligations," White House spokesman Scott McClellan said Wednesday.
Iran has yet to actually suspend uranium enrichment or formally notify the IAEA of its intention to sign the additional protocol.
But its representative to the watchdog, Ali Akbar Salehi, said Tuesday that halting of Iran's work on the nuclear fuel cycle was "probably a matter of weeks." The letter for the IAEA had already been drafted and would be delivered in a "matter of days", he said.