Members of Parliament were today debating whether the UK really has to leave Euratom, the atomic energy organisation, when it leaves the EU. The UK government has already served its intention to leave both bodies at the same time, but in recent days there have been concerns about the possible impact of this. So is it feasible to reverse course on the Brexit process in part?

There are three possible scenarios. First, the government presses ahead with its plan to leave Euratom entirely. One of the planned Brexit bills, on nuclear safeguards, will partially give effect to this intention. The problem here is that there are reportedly enough Conservative MPs ready to rebel on the Euratom issue to overturn the slim combined Tory/DUP majority in the House of Commons, unless they back down following government reassurances.

In the second scenario, the government could make clear its intention that the UK should seek an association agreement with Euratom after Brexit. This isn’t inherently unfeasible, since Switzerland and South Korea have such deals. But the UK would likely be asking for a stronger relationship than those countries have, and the EU side would have to agree to this. There may not be enough time to negotiate it before Brexit Day, and so the UK could enter into a transitional deal in the meantime. This might not be controversial in itself, but it would likely be hard to separate from the issue of a transitional deal relating to the EU, which is far more controversial. There would be a limited role for the EU court, the European Court of Justice, which would upset those who are strongly opposed to any role for this court as regards the UK.

In the third scenario, the government tries to reverse course, and cancel its intention to leave Euratom. This raises multiple questions. Can the EU and Euratom even be separated for this purpose? Although they are legally distinct bodies created by separate treaties, there are still links between them. In particular, the Euratom treaty says that Article 50 of the EU Treaty—the famous legal rule governing withdrawal from the EU—applies to Euratom too.

That could mean that withdrawal from one necessarily means withdrawal from the other, in which case trying to stay in Euratom (but not the EU) is futile anyway. Or it could mean only that the same process applies to leaving either body, but it’s not necessary to leave them both at the same time. Even in that case, staying in Euratom would be awkward, since (as I discuss in more detail here) the Euratom treaty also says that the rules on the EU institutions apply to Euratom. That means the UK would stay part of the ECJ, the European Parliament and the European Commission. Even staying in these bodies for limited purposes will likely upset many of those who voted Leave.

There’s another issue too. Since the Article 50 letter to leave both the EU and Euratom has already been served, could it be withdrawn at all? Would this need the EU’s consent? Can withdrawal be only partial? And any move to withdraw the letter in part opens up a can of worms as regards the certainty of the government’s intention to leave the EU.

The easiest and most sensible course for the government may be to ask for an association agreement with Euratom after Brexit. Yet, as noted above, even that requires potentially difficult negotiations and compromises that could spill over and affect more controversial issues. Euratom could turn out to be a small thread of the Brexit process that, if pulled, causes a greater unravelling.