A small nation with a proud history. A rich economic history. And a bold bid for independence from an over-bearing neighbor. A few weeks ago it was Scotland looking to break away from the United Kingdom. Now it is Catalonia looking to break away from Spain.

The Scottish question has been settled — at least for the time being. But the drive for Catalonian independence is a far bigger threat to the European markets than the Scottish independence movement ever was.

Why? There are three main reasons. First, it is far more likely to happen — while Scotland had little economic incentive to break away, Catalonia has plenty. Second, Catalonian independence threatens the stability of the single currency. And finally, while London was relatively relaxed about losing Scotland, Madrid is not at all relaxed about losing Catalonia. The argument could turn very ugly.

The markets took a while to wake up to the threat posed by a Scottish breakaway, but when they did they wobbled. Likewise, they have not yet woken up to the threat Catalonia poses — but when they do, it could trigger a substantial dive.

The Catalonian independence movement has been gathering strength for many years. But right now, it appears to have reached a tipping point.

Following the Scottish vote, the regional government of Catalonia set its own independence referendum for Nov. 9. The question will be very simple. Do you want to remain a part of Spain or not?

The trouble is, the Spanish government has flatly rejected the right of Catalans to choose. The constitutional court has rejected the vote, and it remains to be seen whether it goes ahead. If it does, and Catalans vote yes, it will be hard to resist granting its independence. After all, the days when people were forced to remain in a state against their democratic will are meant to be long behind us.

In reality, there is no reason why Catalonia should not be perfectly viable as an independent nation, just as there was no reason why Scotland could not prosper on its own either. With 7.5 million people, it would be the 99th biggest country in the world: hardly huge, but there are another 94 United Nations members that are smaller.

It would have a gross domestic product of $314 billion, according to calculations by the OECD, which would make it the 34th largest economy in the world. That would make it bigger than Portugal or Hong Kong, which are perfectly viable by themselves. Its GDP per capita would be $35,000, which would make it wealthier than South Korea, Israel or Italy. There is nothing for anyone to be afraid of there.

And yet, when it looked briefly as if Scotland might break away from the U.K., the pound wobbled, and share prices in London took a hammering. That was despite the fact a Scottish breakaway would have relatively little impact on the wider economy. A Catalan succession would be far more serious for the European economy and markets.

Here’s why.

First, it is far more likely to happen.

For all the nationalist rhetoric, Scotland had relatively little to gain from independence. The U.K. is a relatively successful economy, and while Scotland has been doing reasonably well, it has an aging population and faces declining oil revenues. It was subsidized by the larger country it was part of, and was likely to become more dependent on it as time passed.

Although the numbers are hotly debated, there is plenty of evidence that a wealthy Catalonia subsidizes the rest of Spain. Worse, Spain itself is locked into a dysfunctional currency union, which, despite a minor upturn this year, offers little apart from grinding recession, mass unemployment and rising debt.

It is not hard to argue that Catalonia would do better by itself. In truth, people vote with their wallets — not always, but more often than not. In Scotland, the wallet said “no.” In Catalonia, it is likely to say, at least, “perhaps.” So while the odds were always that the Scots would say “no,” it would be rash to make the same calculation about Catalonia.

Next, independence would be a dramatic event for the whole of the single currency area.

Scotland only shared the pound USDGBP, +0.60% with the U.K.. Catalonia shares a currency EURUSD, -0.53% with the rest of the eurozone. What happens if a section of a member state breaks away? Can it still use the euro, or does it have to re-apply? What happens in the interim period? Who is responsible for its debts, and how are they divided up with the remaining Spanish state?

No one has the faintest idea of the answer to any of those questions. The euro is meant to be irreversible, but if one region breaks away there is nothing to stop others doing so. The euro has enough question marks hanging over it right now. One more is not going to help — and could easily plunge the currency back into crisis.

Finally, breaking up is not going to be easy.

The British government allowed a vote in Scotland, and while Prime Minister David Cameron campaigned against splitting up the union, there was no question that if the Scots decided to take that path they would be allowed to do so. The divorce would be amicable.

Madrid does not feel the same way. What would happen if Catalonia does vote for independence? No one quite knows. There could easily be a long period of hostility as the breakup was negotiated, and a nasty standoff between the two states. That is hardly going to help the stability of an already faltering European economy.

The Spanish bond markets have already started to look uneasy ahead of the vote next month.

But the Scottish experience suggests the markets will only seriously wake up to the threat about a week before the vote takes place. On that reckoning, the first week of November could be a rocky one for the euro — and all the main European bourses as well.