European powers have triggered a formal dispute mechanism over Iran's breaches of key parts of the 2015 nuclear deal - a move that could spell its end. Iran has gradually lifted all limits on its production of enriched uranium, which can be used to make reactor fuel but also nuclear weapons. It has said it is entitled to do so in response to sanctions reinstated by the US when it abandoned the deal in 2018. France, Germany and the UK said they did not accept Iran's argument. The deal saw Iran, which insists that its nuclear programme is exclusively for peaceful purposes, agree to limit its sensitive activities and allow in inspectors in return for the lifting of crippling economic sanctions. President Donald Trump reinstated US sanctions to force Iran to negotiate a new agreement that would place indefinite curbs on its nuclear programme and also halt its development of ballistic missiles. Iran has so far refused. The other parties to the deal - the three European powers plus China and Russia - have tried to keep it alive. But the sanctions have caused Iran's oil exports to collapse and the value of its currency to plummet, and sent its inflation rate soaring. Tensions between Iran and the West have also escalated as a result of a series of attacks in the Gulf region that the US has blamed on Iran and its proxies, as well as a US drone strike in Iraq this month that killed top Iranian general Qasem Soleimani. Treaties and agreements usually have dispute mechanisms to allow one party to challenge another if they think the terms of the deal are being broken. But that is not the situation with the Iran nuclear agreement. The dispute mechanism is there but the moment for invoking it may be long past. That is because one major party - the US - has already abandoned it reimposing crippling economic sanctions against Tehran. For its part Iran has taken a series of steps to breach the deal's constraints. So the deal exists but in a kind of limbo - abandoned or largely abandoned by its two most important signatories. In invoking the dispute mechanism, the Europeans are taking the first formal step towards writing its obituary. They insist that they will stand by it for as long as it exists and that they want a better deal - one that the US can support. But it is very hard to see Iran accepting a more restrictive agreement that will include constraints on its missile programmes and maybe also its regional behaviour. And it is equally hard to see President Trump lifting the sanctions - not least when he believes - with protests against the regime under way - that his maximum pressure campaign is working. The European decision to invoke the dispute process may be a final bowing to the inevitable. The nuclear agreement is in a critical condition and slowly slipping away.