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Britain, Germany, and France said Tuesday they had “been left with no choice” but to finally challenge Iran over its breaches of the 2015 nuclear accord, after Tehran announced it was abandoning limits on the enrichment of uranium altogether.

The European countries announced they had triggered the “dispute resolution mechanism” outlined in the landmark deal, which will bring their grievances before the other signatories — Iran, Russia, China, and the European Union — to try to find a resolution. If they can’t reach an agreement, Iran, whose economy is already suffering under U.S. sanctions, could face the return of crippling international measures.

Under the deal, which was intended to stop Iran developing a nuclear weapon, Tehran agreed to limits on its nuclear program in return for the lifting of international sanctions.

But since the U.S. withdrew in 2018 and reimposed its own sanctions, the Islamic Republic has gradually announced that it is walking away from various restrictions imposed by the agreement. Until now, European nations have tried to hold the weakened agreement together, but their latest statement signals that they have had enough of Iran’s breaches — and that Tehran could be hit with more devastating sanctions if it fails to get back in line.

In Iran’s latest rollback of the deal on Jan. 5 — just days after a U.S. drone strike killed a top general — it announced that it was abandoning all limits on the production of enriched uranium, which can be used to make reactor fuel as well as nuclear weapons. Under the deal, Iran’s enrichment had been limited to 3.67 percent, to prevent the country acquiring enough 90-percent enriched uranium threshold to build a weapon.

“Iran will continue its nuclear enrichment with no limitations and based on its technical needs,” said an announcement in state media.

The statement said the country would return to its commitments when it once again derived benefits from the deal — in other words, when the U.S. lifts freezes on oil sales and assets.

Instead, the latest rollback of the deal finally forced the European nations’ hand. In response Tuesday, the three countries — referring to themselves as the “E3” — said they had worked hard to save the deal following the U.S. withdrawal, and still sought to preserve it.

READ: Iran has already hacked the U.S. at least 4 times — and could do it again

“However, in the meantime, Iran has continued to break key restrictions set out in the JCPoA. Iran’s actions are inconsistent with the provisions of the nuclear agreement and have increasingly severe and non-reversible proliferation implications,” said the statement, using the initials for the deal’s formal name, the Joint Comprehensive Plan of Action.

“We do not accept the argument that Iran is entitled to reduce compliance with the JCPoA.”

Meanwhile, British Prime Minister Boris Johnson, a strong ally of U.S. President Donald Trump, said that the best solution would be to strike a new deal, with U.S. buy-in.

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In an interview with the BBC Tuesday, Johnson said he acknowledged the flaws in the 2015 deal, but that the priority was to stop Iran developing a nuke.

“If we’re going to get rid of it, let’s replace it and let’s replace it with the Trump deal,” he said.

“President Trump is a great dealmaker, by his own account. Let’s work together to replace the JCPoA and get the Trump deal instead.”

The European challenge over the nuclear deal comes as Iran’s government is under strain, both at home and on the international stage. In November, protests broke out around the country in response to the impact of significant gas price hikes, prompting a violent response by security services in which hundreds of demonstrators were killed.

READ: Here’s what happened in Iran while the government strangled the internet

The government has faced even greater pressure since the U.S. assassinated top general Qassem Soleimani in a drone strike earlier this month. Hours after Iran fired on U.S. military bases in Iraq in retaliation, it accidentally shot down a passenger jet leaving Tehran, killing all 176 people on board. The government’s admission of guilt for the disaster Saturday, after days of denials, has drawn protesters back into the streets and prompted widespread condemnation of the regime.

READ: Witnesses say Iranian security forces opened fire on protesters

Cover: Boris Johnson visits Stormont. Prime Minister, Boris Johnson, speaking in the Great Hall, Stormont, Belfast, whilst on a visit to meet the leaders of the restored powersharing government. Liam McBurney/PA Wire URN:49550458 (Press Association via AP Images)