And three senior lawyers with extensive knowledge of European and nuclear law told BuzzFeed News that while the legal question might not be settled, it was difficult, practically speaking, to see a way Britain would leave one without leaving the other. The decision to leave Euratom is most likely no longer in British hands, and possibly never was.

"In reality we couldn’t have triggered Article 50 without leaving Euratom as well," Catherine Barnard, a professor of EU law at the University of Cambridge, told BuzzFeed News. That's because Article 106a of the Euratom treaty makes it clear that several EU treaty provisions, including Article 50, apply directly to Euratom as well as the EU.

"Even if [MPs] voted to reverse the decision to authorise the PM to leave the EU and Euratom," she said, "which they gave when they approved the Article 50 withdrawal bill, there are two major problems. One, we've said we're leaving and Article 50 makes no provision for changing your mind. And two, Euratom membership is intimately linked to EU membership through shared institutions" – such as the parliament and council of ministers.

Paul Bowden, a partner at the law firm Freshfields Bruckhaus Deringer and programme leader of the International School of Nuclear Law at the University of Montpellier, told BuzzFeed News that legally, "I don't believe that there is a clearly settled answer."

Practically, however, he said that doesn't matter, because Theresa May's letter triggering Article 50 in March made it clear Britain would also leave Euratom. So the legal question of whether it would have been possible has been bypassed. The two have been triggered together, that has been accepted by the European bodies, and "I don’t think there can be any serious legal question as to the validity of the Article 50 notice insofar as it applies to Euratom."

If both Britain and the remaining 27 EU members wanted to revoke the Article 50 notice, or the part of it that applied to Euratom, that might happen, but Britain can't do it unilaterally. "With willing partners anything is possible," said Bowden, "although that would require very careful negotiation in its own right. But I don't think the 'oh we've changed our mind, we didn't mean it' option is available."

And there is no realistic way that the necessary agreements will be in place in time for March 2019, when Britain finally leaves. "No! It's not realistically possible," Barnard said emphatically. "If we were just doing this and nothing else, maybe. But even then it involves major infrastructure issues, and we're dealing with dangerous stuff. And it's being done in amongst all the other activities that need to be done."

She also pointed out that in fact we have much less time than that anyway. "The EU parliament would need to sign off on any deal," she said. "And they say they need several months to do it. So negotiations will have to be done by September 2018. We've actually only got just over a year."

It is "critical", however, that some sort of deal is reached, according to Bowden, in order to meet the UK's commitments under international law to things like the Nuclear Non-Proliferation Treaty and the International Atomic Energy Agency. "I find it very difficult to see how the UK can continue to meet those obligations without a very strong continuing measure of equivalence between our regulatory standards and those of Euratom," he said.