The way they do this in both Scotland and the separatist regions of Spain is by insisting that membership in the European Union would pass automatically to the successor state. The opponents of secession, however, argue that there's nothing automatic about it.
The arguments are not just directed at the home audience. Last month, when Scotland's First Minister, Alex Salmond, agreed the terms for the 2014 referendum with the British Government, Spanish Foreign Minister Jose Manuel Garcia-Margallo declared that an independent Scotland would not automatically be an EU member, and that any one of the 27 EU member states could veto it.
"In the hypothetical case of independence," he said, "Scotland would have to join the queue [for EU membership] and ask to be admitted, needing the unanimous approval of all member states to obtain the status of a candidate country." The European Commission president, Jose Manuel Barroso, also said an independent Scotland would be seen as a new stat d would have to apply to join.
This was disputed by Alex Salmond, who knew his chances of winning the 2014 referendum were nil if the Scots believed they were voting to leave the EU. For months he insisted he had sought the opinion of his government's law officers, who had confirmed Scotland would inherit EU membership automatically, and would not even have to adopt the euro. Alas, he was lying.
Late last month, it became known that Salmond had not asked for the law officers' opinion. Now he has been forced by public opinion to pop the question - and he may not like the answer.
An even bigger defeat for Salmond came in his negotiations with British Prime Minister David Cameron, where he had to agree that the referendum would ask a simple yes-or-no question: in or out? This goes against the instincts of all separatist leaders, who prefer a fuzzy, feel-good question that doesn't mention the frightening word "independence".
The most famous formulation of this question was in the 1995 Quebec referendum on secession from Canada: "Do you agree that Quebec should become sovereign after having made a formal offer to Canada for a new economic and political partnership within the scope of the bill respecting the future of Quebec and of the agreement signed on June 12, 1995?" Not exactly clear, is it?
That referendum was very close, but in 2000 the Canadian federal government passed a law generally known as the "Clarity Act". It said that negotiations between the federal government and any province on secession should only follow "a clear expression of the will of the population of a province that the province cease to be part of Canada".
This requirement would not be met, it added, if the referendum question "merely focuses on a mandate to negotiate without soliciting a direct expression of the will of the population of that province on [independence]," or if the question "envisages other possibilities ... such as economic or political arrangements with Canada, that obscure a direct expression of the will of the population on [secession]."
This law drastically reduces the likelihood the separatists could win any future referendum in Quebec, and it's obviously what Cameron had in mind in his negotiations with Salmond on the Scottish referendum. As for Catalonia and Euskara, the national parliament in Madrid must approve of any referendum on separation, and the Spanish Government has made it abundantly clear it has no intention of doing that.
So it's mostly just hot air and hurt feelings, really.