Fears Iran will start another war have skyrocketed after European countries triggered a "snapback mechanism" which could provoke Tehran into restarting its nuclear programme.
The UK, France and Germany, known as the E3, have given Iran 30 days to "address the international community's serious concerns over its nuclear programme".
If it fails, "targeted UN sanctions focused on tackling Iranian nuclear proliferation" will return.
Iran's nuclear hopes were dented months ago when Donald Trump bombed three of Iran's main atomic sites in a bid to stop the so-called "12-day war" between Iran and Israel.
And now, a joint statement from the three European nations has vowed: "We remain determined that Iran must never acquire a nuclear weapon."
But in a letter to the EU, Iran's Foreign Minister claimed the three countries have "no legal jurisdiction" to restart the sanctions.
Iran could face more sanctions unless it enters diplomatic talks with European nations
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Iran continues to claim its nuclear facilities are for civil purposes
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He added that Iran's position was being supported by Russian and China - but that his nation was prepared to resume "fair and balanced" negotiations if other parties showed "seriousness and goodwill".
The statement from the E3 also suggested "there remains a clear diplomatic pathway forward, should Iran choose to take it" tensions between the two sides seem to remain high.
Two weeks ago, the trio said they would resume sanctions unless Iran agreed to a "diplomatic solution" by the end of August.
In response, Iran's Foreign Ministry said this would "seriously undermine" its ongoing work with the International Atomic Energy Agency (IAEA).
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The UK, France and Germany have threatened to invoke the snapback
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It called the move a "provocative and unnecessary escalation" which "will be met with appropriate responses".
But the E3 warned that Iran had stopped providing the IAEA access to its nuclear sites.
In a 2015, a UN-backed deal between Iran and the US, UK, France, Germany, China and the EU saw sanctions lifted from Iran in exchange for curbs to its nuclear programme.
The deal included the threat of snapback provisions and sanctions to be brought in if Iran fails to fulfil its nuclear commitments.
David Lammy said the sanctions relief could be extended, provided Iran meets certain conditions
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Now that the E3 has sent the letter, the UN Security Council has 30 days to decide whether sanctions should be reapplied to Iran.
Foreign Secretary David Lammy said the UK, alongside its European allies, has offered to extend the relief from sanctions, provided Iran meets certain conditions.
However, he said "no substantive effort" had been made by Iran, and the country "consistently failed to provide credible assurances on the nature of its nuclear programme".
Urging the council to reject the snapback, Iran's Foreign Ministry claimed it had shown "the utmost restraint and steadfast commitment" to diplomacy.
The United States has welcomed the snapback decision, agreeing to work with the E3 to enact it
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The United States has welcomed the snapback decision, agreeing to work with the E3 to enact it.
A spokesman for the US Secretary of State, Marco Rubio, said: "At the same time, the United States, remains available for direct engagement with Iran - in furtherance of a peaceful, enduring resolution to the Iran nuclear issue."
Iran continues to claim its nuclear programme is not working towards weapons, but rather civil matters.
However, the IAEA has said it do not believe Iran's purposes to be purely peaceful.