In the stormy Westminster debate on Brexit, one idea that is gaining ground each day is that the UK should seek to delay its departure from the EU. The SNP is only the latest to call for extra time: the party said on Monday it wanted an extension of article 50 beyond March 2019 so that the government could “go back and negotiate a better deal”.

The shadow Brexit secretary, Keir Starmer, thinks the EU and the government could be persuaded to extend article 50 if MPs reject Theresa May’s deal on 11 December. Meanwhile, prominent supporters of a second referendum, such as the Conservative former cabinet minister Justine Greening, think the exit clause could be extended until the end of July 2019 to allow time for another vote.

In 115 days’ time, if nothing else happens, the UK will leave the EU. Like everything else in Brexit negotiations, extending article 50 is not simple, and the EU holds all the cards.

What does the law say?

The EU treaty is clear: a country’s exit negotiations can be extended with the unanimous agreement of the remaining member states. But article 50 is otherwise silent about procedure or timing.

Can the UK rely on the EU agreeing to an extension?

Since the day the British government lit the fuse on article 50, EU officials have suggested a few weeks’ extension would be possible – in certain circumstances. The point of extra time would be to finalise ratification and tie up legal niceties (such as translation of texts), to avoid an accidental no-deal. Now, the talk of the Brussels coffee bars is whether the UK could get to delay Brexit because of a political shift, such as a second referendum. In this case, extension is not ruled out but runs into the hurdle of European elections due on 23-26 May 2019.

How long could talks be extended?

France was among the first to insist Brexit must be over before the European elections. This is the Brussels consensus, although no formal discussions have ever taken place. Officials think it would be legally impossible for a country to be an EU member state without participating in elections. To hear otherwise from the UK, which has long been the source of complaints about the EU’s “democratic deficit”, sounds a bit rich to some EU officials, who fear that any British citizen could take the EU to court for denying them representation. “Legally, it’s easier to postpone the European elections [than have the UK in without taking part] and that’s absurd,” one senior source said.

Nevertheless, some diplomats speculate that it could be possible to engineer an extension until mid-summer, before the first sitting of the European parliament, which is usually in July. This reflects a desire to find a pragmatic answer if the UK needs more time, but it is a minority view, not backed up by formal legal opinion. The majority insist the UK would still need to take part in May’s elections, even if the extension expired in June or July.

Can the UK revoke article 50?

Simpler than extension, could the UK simply cancel the article 50 notification? That is the nub of a case before the European court of justice on “whether, when and how” article 50 notification “can unilaterally be revoked” by the UK. Remainers, as well as the Scottish politicians who brought the case, received a massive boost on Tuesday, when a senior jurist at the ECJ concluded that article 50 could be withdrawn by the state that triggered it. Preventing the revocation of article 50 could mean the “forced exit” of a country that was still an EU member state, said an ECJ advocate general, whose advice is usually followed by the court.

The ECJ’s final ruling is not expected until the end of this month, so MPs will not have a definitive answer when they vote on May’s deal.

But the preliminary opinion suggests that if the UK chose to revoke article 50, it would be a decision to cancel Brexit, not gain extra time for negotiations – the UK would be expected to withdraw its notification in “good faith”. In this way, the advocate general seeks to tackle the arguments of EU lawyers, who contended that allowing a state to cancel article 50 would leave the union in “endless uncertainty”, allowing a country to stop and start the countdown clock to get better terms.

Leave and then return?

Faced with the looming deadline, some think the UK would be better off leaving and trying to return during the Brexit transition. In other words, forget article 50 and focus on article 49, the EU treaty provision on joining the club. One member of the European parliament’s Brexit steering group, the Belgian MEP Philippe Lamberts, has counselled remainers to adopt this strategy – assuming the option to return won in a referendum or general election.

EU officials are less convinced a return would be so straightforward or quick. The UK’s readmission would require the consent of 27 countries, some led by populist anti-EU governments and coalitions. The UK would not regain the special deal it has now, including a generous rebate and opt-outs on other EU policies. “It is a high-risk strategy,” one senior official said. If that was the plan, “they really have to stop the train before it leaves the station”.