EU countries warn Britain on Brexit: You'll pay if you leave us

Brussels could offer Britain one of the three existing models of varying closeness and mutual obligation — the bloc’s arrangements with Norway, Canada and the World Trade Organization — rather than offer to negotiate something new, said Charles Grant, the director of the Centre for European Reform, a London research institution.

The effect on the bloc of a British departure would be threefold, Mr Grant said.

First, he said, a British withdrawal would be a big boost to nationalist, anti-European Union movements, with Marine Le Pen, the leader of France’s far-right National Front, already comparing a British exit to the fall of the Berlin Wall.

“It’s not that the right will win power, but they’ll feel they have history with them, and pro-European elites in government will be on the back foot and afraid of moves toward more integration,” Mr Grant said. “Federalism would be dead, and there would be no more referendums and treaty changes for generations, so it would be a new period of national power and not the federal future the European Commission wants.”

Second, he said, without the counterweight of Britain, “the German problem becomes more acute.” Rome, Paris and Warsaw fear that without Britain as a countervailing force in the bloc, Germany would become too powerful. The Germans themselves fear that an anti-German alliance would form.

Third, he said, the European Union without Britain’s free-market influence would be more French in its economic policy outlook and more protectionist, with little impetus for free-trade deals or for the extension of the single market to services.