The UK, France and Germany have formally notified the UN that they have triggered the restoration of sweeping UN sanctions against Iran, giving Tehran 30 days to make concessions on access to its nuclear sites or face deeper worldwide economic isolation.

UK officials said the decision had not been taken lightly and there had been intensive diplomacy to try to avert this step. The officials stressed there was still room for some last-ditch diplomacy before the sanctions “snapback” comes into force in 30 days’ time. The annual high-level UN general assembly in September is likely to host more intensive diplomacy over the situation with Iran.

They added that Iran had been in significant non-compliance with the terms of the 2015 nuclear deal since 2019, saying Tehran had been given ample opportunity to become compliant, but had either been unwilling or politically unable to act.

The International Atomic Energy Agency (IAEA) quit Iran after Israel bombed its key nuclear sites, and has been unable to renegotiate a return to assess the state of those sites. Iran has allowed the IAEA inspectors to visit the Bushehr site where refuelling is due to occur.

The UK foreign secretary, David Lammy, told the Iranian foreign minister, Abbas Araghchi, of the move by the three European countries in a call on Thursday morning.

According to a statement from his ministry, Araghchi said in the call that Iran would “respond appropriately to this illegal and unjustified action by the three European countries in order to protect and guarantee its national rights and interests”.

The effect of the snapback will convert many existing, largely US and European, sanctions into UN-wide sanctions requiring observance by Russia and China. Some sanctions on the Iranian arms industry that are not currently in force will be restored. In addition, the US, as a permanent member of the UN securitycouncil, will in future have a veto over the lifting of the UN sanctions, a power it currently does not have.

The three European powers had to act now before the nuclear deal expired on 15 October and their power to reinstate sanctions ended.

Justifying the decision, UK officials said the 2015 nuclear deal allowed Iran to store only 300kg of uranium enriched up to 3.67%. Instead the stockpile was 45 times larger than this limit, with much of the uranium over the purity levels set by the 2015 agreement. Iran also had a 400kg stockpile of unaccounted highly enriched uranium.

The officials said: “We have bent over backwards to reach an agreement but there simply has not been enough of a response on the Iranian side, including eight rounds of talks this year.

“The most basic legal requirement, for the UN nuclear inspectorate to be able to access Iran’s nuclear sites, has not been met. Bluntly, we have run out of time. We cannot allow the Iran nuclear file to fall off the agenda or for sanctions to be permanently lifted while Iran’s nuclear programme escalates and IAEA access is denied in clear breach of its obligations under the nuclear non-proliferation treaty.”

They insisted the snapback step was legal and proportionate, and the UK remained committed to a diplomatic solution.

They added the UK had in June suggested a six-month delay in enforcing the snapback on the condition that Iran allowed the IAEA back into Iran.

The officials said Iran had stopped implementing the additional protocol, allowing enhanced inspections of its nuclear sites.

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Some experts thought Iran might respond by withdrawing altogether from the non-proliferation treaty at the most extreme, or hardening its opposition to the return of the IAEA.

Although the Iran nuclear file is not at the top of the White House agenda, Iranians fear it is likely to attract Donald Trump and Israel’s attention soon, raising the possibility of further attacks.

Iran is divided between those who want to accept US demands in return for the lifting of sanctions and those unafraid of further confrontation.

The Iranian leadership, caught by surprise by the June attacks, has been reorganising its defence and security leadership, and it is possible that it has not had time to reach an internal consensus.

The Iranian parliament, dominated by conservatives, has set strict conditions for the return of the IAEA, but these can be overridden by Iran’s national security council.