Euro-skepticism has remained a current in British politics, long after Britain put aside centuries of anti-European sentiment to join the Common Market in 1973 and to become a member of the more highly integrated European Union 20 years later.

Until now, periodic spasms of parochialism did little lasting harm. But since the European debt crisis and recession, there has been growing sentiment across the British political spectrum for leaving the European Union. As Roger Carr, the leader of Britain’s biggest business group, the Confederation of British Industry, rightly warned on Monday, this latest trend needs to be taken seriously and strongly countered by pro-European business and political leaders. Leading political figures expect a national referendum to be called on continued British membership in the European Union, probably for 2015. If it were held today, an Opinium/Observer poll suggests, the anti-European side would prevail, with a large majority of Conservatives, a plurality of Labour voters and a significant minority of Liberal Democrats voting for a British exit.

Leaving the union would be a grave mistake, sacrificing Britain’s best hopes for a brighter economic future to half-baked longing for the simpler days when the British ruled an empire and had less need for European trade. There’s plenty of time for pro-Europe leaders to make that case. But they must heed Mr. Carr’s advice and speak up.

Twenty-seven countries now belong to the European Union, with several more nations like Iceland, Serbia and Turkey hoping to join. Even without further expansion, the union accounts for almost 50 percent of British trade and is its largest trading partner.