Among those who manage gobs of money, the possibility that Britain might actually disavow the European Union seemed until recently like a remote and even outlandish possibility.

But about a week before voters go to the polls to determine their future, masters of finance are suddenly absorbing the prospect that Britain might really walk, unleashing anxiety and uncertainty throughout the global economy.

Like local responders readying sandbags as a hurricane menaces their shores, financial industry overseers have been quietly drawing up contingency plans while surveying the expensive havoc a so-called Brexit is already wreaking. Central bankers from London to Washington have been monitoring the tempest while making preparations to unleash credit should markets seize with fear.

Angst has seeped into the calculations. As investors digest the possibility that the largest marketplace on earth may be days away from a messy alteration, they have been yanking money out of riskier storehouses like stocks and putting it into safer instruments like bonds. The British pound and London stocks have been falling in frenzied trading.