This longstanding discomfort about getting too close to Europe now appears to be morphing into a dangerous stampede for the exit. The Parliament recently voted down Cameron’s already stingy proposal on the E.U. budget. Pressed by Euro-skeptics in his own party, Cameron is edging toward committing to a popular referendum on membership in the E.U. (Recent polls reveal that 48 percent of voters favor withdrawal, with 31 percent against).

Cameron himself favors Britain’s continued membership in the E.U., but only if the country is able to negotiate a more attenuated relationship. He wants to consolidate a “two-speed” E.U. in which members outside the euro zone would contribute less to the Union budget and form their own caucus. He has also indicated that his government will repatriate control of key aspects of social policy and law enforcement from Brussels.

These moves are meant to provide the British the breathing room they need to stay put in the E.U. But rather than consolidating Britain’s place in the E.U., Cameron’s course is likely to precipitate its defection. Of Europe’s 27 members, 10 are outside the euro zone. Most of Britain’s companions in this outer circle, however, plan to eventually join the euro zone, which would leave Britain as the odd man out — absent from what will become the E.U.’s main decision-making venue. The British would hardly countenance remaining in a Union in which their voice would be so marginalized.

Britain is also set to pay a heavy economic price for standing aloof. The euro zone is fashioning banking and fiscal unions that would give Europe’s single market an integrated financial sector. London’s position as one of the world’s financial centers could be undermined as bankers and traders move to the euro zone to take advantage of its common rules and single currency. Britain’s hostility toward the E.U. would only mount.

Britain’s drift from Europe is also a geopolitical dead end. Britain has long served as a bridge between the United States and Europe, but London cannot remain an important partner on matters of European defense should it become a bit player within the E.U. Britain would resign itself to a no-man’s-land if it abandoned its attachment to the E.U., weakening the Union as well as Europe’s tether to the United States.