Brexit may look bad for sterling, but for the euro it looks really ugly. Here we are in the middle of global economic decrepitude, miserable inflation, the market madhouse, central bank frailties and a migrant crisis. Now comes a four-month debate on whether the people of the EU's third most populous member thinks the whole eurofederal shindig is worth the candle anyway.

If the Brits end up concluding it isn’t, that will surely prompt others to bang on the exit door. Even if the UK votes to remain, there will be demands elsewhere for similar “special status” deals.

To his critics, David Cameron’s Brussels bartering yielded precious little. But they aroused pique and envy on the continent.

All that makes the euro’s performance this spring more significant than that of sterling. The euro is already off 1 per cent this week. If currency wars are Mario Draghi’s thing, then the European Central Bank president would normally welcome a weaker euro to stimulate the eurozone’s economy and help its struggle with low inflation.

But there is good euro depreciation and bad euro depreciation. The former happens when a strong US economy drives a healthy dollar upward; the latter when the eurozone economy turns sour, the market turns on the ECB and a crisis turns into an existential problem.

Right now, the euro is falling for the wrong reasons. Business growth and expectations are flagging, Mr Draghi’s negative interest rates policy is being picked apart and Brexit is a scenario too scary for Brussels to contemplate.

Watch the euro against 10-year Bund-Treasury spreads, suggests Société Générale strategist Kit Juckes. Moves in the spread shift the euro. But if that correlation weakens, and the euro continues to fall without a corresponding rise in Treasury yields, the euro will start to drift and head for a crisis of confidence — as will the single currency project.

Been here before? A year ago, Grexit was the black swan risk, and faded away. Today feels different to 12 months ago when monetary divergence was all the rage, Mr Draghi was in his pomp, and China and oil had yet to muddy the waters. The UK has long been a semi-detached EU member, and the Brexit threat could not be put off indefinitely. But boy, has it come at a rough time for the euro.