T HE MORNING after Theresa May’s Brexit deal was defeated, 27 fed-up ambassadors from across the European Union gathered in Brussels for yet another meeting on Brexit. But, for once, there was something to discuss: whether to grant an extension to the Article 50 deadline.

Delaying Brexit beyond March 29th requires the unanimous approval of heads of government at the EU summit, set for March 21st. The parameters of any extension are already becoming clear. Britain must offer a “credible” reason, in the words of Donald Tusk, the European Council president, which were echoed by leaders from across the block. And if it wishes to hang on as a member beyond the European elections at the end of May it must elect a new batch of MEP s.

But what counts as “credible”? A long delay so that Britain could have a general election or a second referendum would almost certainly cover it, say diplomats and EU officials. Likewise, a fundamental change in Britain’s Brexit strategy.

A short extension to avoid Britain crashing out comes with little cost to the EU . A no-deal exit may be worse for Britain, but it is hardly a good outcome for the EU either, point out diplomats. Most predict that the EU will happily offer a short extension until the European elections.

Mr Tusk has urged members to be “open” to a longer delay. But there is reluctance to give extra time purely so that Mrs May can continue banging her head against a wall, or to allow MP s to propose ideas already dismissed by Brussels. France is firm on this, calling any such extension “totally unacceptable”. It is not alone. “If it is the same old stuff, why would we give any extension?” asks one commission official.

Other than demanding unanimous agreement, Article 50 is silent on what terms must be offered. EU leaders can be as strict or as lenient as they like. An extension that bleeds into the EU ’ s next budget period would be too complicated, say some diplomats, suggesting a natural upper limit of 2020. It may be a strict one-off or more open-ended. Whatever happens will be a nakedly political decision taken by 27 leaders, all with their own national concerns, round a table in Brussels.

Patience is not infinite. Throughout the negotiations, heads of government have tended to be tougher than their ministerial underlings. All 27 leaders face the ballot box in the upcoming elections, where a gamut of populists and radicals are expected to sweep up seats, turning the European Parliament into a political zoo. A firm line with Britain may help some EU leaders fend off this domestic populist menace.

When it comes to the European elections, the main concern is legal rather than political. If Britain is still in the EU by the time elections roll around, the country will be obliged to elect new MEP s, insist diplomats. Any doubts about the legality of a European Parliament constituted without British MEP s while Britain is still a member risks legal challenge, destroying the quarantine maintained between Brexit and other policy areas in the EU .