The UK, France and Germany have begun proceedings against Iran after triggering a dispute mechanism in the 2015 nuclear agreement.
The European move comes amid escalating diplomatic tensions between the west and the Islamic republic.
What is the deal?
The Joint Comprehensive Plan of Action came about in 2015, when Western governments believed Tehran was on the brink of creating a nuclear weapon.
Three European countries - the E3 of the UK, France and Germany signed the deal, along with Russia, China,the US and of course,Iran.
This would attempt to ease international sanctions on Iran in exchange for restrictions on Tehran's nuclear programme.
Was it successful?
Iran has not developed a nuclear weapon, the key aim of the agreement.
But the agreement was disrupted when the US withdrew in 2018, with President Donald Trump claiming it gave Tehran access to too much money and did not stop its "malign behaviour" in the Middle East.
The US reimposed sanctions as part of Mr Trump's "maximum pressure" strategy which has hit Iran's economy. The country then stepped back from some of its commitments.
What happens now?
The deal has effectively been weakened after the UK, France and Germany began the dispute resolution mechanism.
It was triggered by Iran declaring on January 5 that it was discarding the last component of the limitations imposed by the deal and no longer faced "any operational restrictions" on its nuclear programme.http://players.brightcove.net/1348423965/default_default/index.html?videoId=6121419991001Iran video appears to show missile striking Ukrainian plane
Under the dispute resolution mechanism, Tehran will be given 15 days to respond, a term which can be rolled over indefinitely - so the move is aimed at buying time to reach a diplomatic solution.
Will the deal survive?
The UK and its allies felt they needed to prevent the deal from becoming ineffective, by moving away from its main-purpose, to limit Tehran acquiring weapons-grade nuclear material.
There is little hope in Whitehall that merely triggering the dispute process will cause an immediate change in behaviour from Tehran.
The process could result in a return to the UN Security Council and the reimposition of sanctions, something the UK wants to avoid.
What are the alternatives?
The JCPOA is the only deal on the table and the UK and its allies have publicly confirmed their commitment to it.
But Boris Johnson has also urged Mr Trump to think up an alternative proposal he would be willing to support, saying: "Let's work together to replace the JCPOA and get the Trump deal instead".