EC Vice President Olli Rehn said the British government could spend its political capital better working to reform the European Union rather than undermining regional unity.
"For 40 years now, Britain has been stronger thanks to its membership of the European Union," he said. "(It's been) more dynamic economically, more equitable socially (and) more influential in world affairs."
British Prime Minister David Cameron in January said frustration with EU policies has grown and the British would "have their say" on EU membership in a 2015 referendum.
Eurostat, the statistical arm of the European Union, reported Friday that unemployment was at 11.9 percent in January, an increase of 0.1 percent from December.
Nevertheless, Rehn said trade measures under consideration with Washington may help turn the tide for the eurozone.
"We have before us many more far-reaching choices, in the eurozone and in the wider European Union," he said. "In that context, I believe it is firmly in Britain's interest to use its energy for reforming Europe rather than seeking to undo our community, which would leave us all weaker."