Iran has said it will breach on 27 June the limit on its stockpile of enriched uranium that was capped under a 2015 nuclear deal with world powers.
The country announced last month that it would quadruple its production of low-enriched uranium in retaliation for sanctions reinstated by the US.
Enriched uranium is used to make reactor fuel but also nuclear weapons.
European powers have warned Iran they will have no choice but to reimpose sanctions if the deal is violated.
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The UK, France and Germany have tried to stop the accord from collapsing since US President Donald Trump abandoned it last year.
But Iran has complained that they have failed to protect its oil and banking sectors from the effects of the US sanctions. They have triggered a collapse in economic growth, pushing the country into a deep recession.
In early May, after Mr Trump ended exemptions from US secondary sanctions for countries still buying Iranian oil, Iran said it was scaling back its commitments.
This included no longer complying with caps on its stockpiles of enriched uranium and heavy water – set at 300kg and 130 tonnes respectively – and halting sales of surplus supplies overseas.
It also gave the European powers until 7 July to protect Iranian oil sales. If they fail, it will suspend its restrictions on uranium enrichment and halt the redesign of its heavy-water reactor, whose spent fuel would contain plutonium suitable for a bomb.
Iran nuclear deal: Enriched uranium limit will be breached on 27 June}