Abbas Araghchi, who signed the letter with his Russian and Chinese counterparts at a foreign ministers’ summit in Tianjin in China, said the powers were united in condemning Europe's "politically destructive" move.
The snapback mechanism, created under UN Security Council Resolution 2231, allows any signatory to the now mostly lapsed 2015 Joint Comprehensive Plan of Action (JCPOA) to restore previous UN sanctions if Iran is judged to be in major violation.
Once invoked, sanctions return automatically after 30 days unless the Council votes to extend relief. The provision expires in October 2025.
On August 28, Britain, France and Germany formally triggered the process, citing Iran’s accumulation of highly enriched uranium.
The European powers said they were prepared to delay enforcement if Iran resumed cooperation with UN inspectors and engaged in negotiations.
But the three Eurasian powers were categorical in their rejection of the move.
"The UN Security Council cannot proceed on the basis of the communication submitted by the E3 and should consider it null and void," they wrote in the joint letter according to a picture of the document shared by Araghchi.
"We strongly urge the members of the UN Security Council to reject the claims of France, Germany, and the United Kingdom on allegedly invoking the 'snapback' mechanism and reaffirm their commitment to the principles of international law and multilateral diplomacy," they added.
Araghchi blamed the United States and Europe for undermining the deal.
“It was the United States that first violated the JCPOA and Resolution 2231,” he wrote on X, referring to its 2018 exit from the agreement during President Donald Trump''s first term.
“Europe, instead of fulfilling its commitments, chose to align with unlawful sanctions," he said. “Countries that fail to meet their obligations have no right to enjoy the benefits of an agreement they themselves have weakened. The credibility of multilateral diplomacy can only be preserved on this logic.”