"Due to the impacts of America's exit from the JCPOA and the European countries' failure to fulfil their commitments under the JCPOA and the communiques of the joint commission, the Islamic Republic of Iran will suspend all of its JCPOA commitments with regards to nuclear research and development," the letter stated, according to Foreign Ministry spokesman Abbas Mousavi.
In a statement on Thursday night, Mousavi said the third step was Tehran's response to "widespread and continued violations" of the nuclear deal ever since the US dropped out of it 16 months ago in May 2018.
He noted that Iran's right to respond was secured in the Article 36 of the JCPOA, which defines mechanisms for both sides to deal with non-compliance.
According to Mousavi, Iran would soon inform the International Atomic Energy Agency (IAEA) of the technical and operational aspects of its decision to suspend its Riamp;D commitments.
The Iranian diplomat added that all of Tehran's moves in reducing its JCPOA commitments were reversible once the remaining signatories -- the UK, France, China, Russia and Germany -- go back to fully implementing the deal.
Mousavi also said that as a sign of "good will" Iran would continue negotiations with the remaining parties to achieve the desired results.
In response to the development, European Commission spokesman Carlos Martin Ruiz de Gordejuela said Thursday that the decision was "inconsistent" with the JCPOA, asking Iran to "to reverse these steps and refrain from further measures that undermine the nuclear deal", reports Press TV.
France's Foreign Ministry asked Iran to avoid decisions that it said would undermine the nuclear agreement.
The UK Foreign Office also voiced concern, calling the decision by Iran "disappointing".
Iran announced the first stage of its countermeasures in May by increasing its enriched uranium stockpile to beyond the 300 kg cap set by the agreement.
It took the second step two months later, when it started to enrich uranium to purity rates beyond the JCPOA limit of 3.76 per cent.