For Britain and the European Union, they are words that expose big philosophical differences: "Ever closer union."

The phrase, baked into the EU's 1957 founding treaty, remains a fault-line between Britain and its European neighbors.

On Tuesday, the EU's six founding nations — including Italy, Germany and France — stressed their commitment to "an ever closer union among the people of Europe."

The declaration by foreign ministers is awkward for British Prime Minister David Cameron, who is pushing for changes to Britain's relationship with the EU ahead of a referendum on whether to remain in the bloc or leave.

British Euroskeptics seized on the declaration as proof that the EU aims to become a European super-state. The tabloid Daily Express said Wednesday it could leave Cameron's negotiation attempt "in tatters."