Voters in France, Italy and the Netherlands are demanding their own votes on European Union membership and the euro, as the continent faces a "contagion" of referendums.
EU leaders fear a string of copycat polls could tear the organisation apart, as leaders come under pressure to emulate David Cameron and hold votes.
It came as German business leaders handed a considerable boost to the Leave campaign by saying it would be "very, very foolish" to deny the UK a free trade deal after Brexit.
Markus Kerber, the head of the BDI, which represents German industry, said that 1970s-style trade barriers would result in job losses in Germany.
"Imposing trade barriers, imposing protectionist measures between our two countries - or between the two political centres, the European Union on the one hand and the UK on the other - would be a very, very foolish thing in the 21st century." In Italy, the anti-establishment Five Star movement on Tuesday declared it would demand a referendum on the euro. The party wants the euro to be split - one for the rich north and one for the south.
Beppe Grillo, the party's leader, has called for a full referendum on EU membership. He said: "The mere fact that a country like Great Britain is holding a referendum on whether to leave the EU signals the failure of the European Union."
Five Star won 19 out of 20 mayoral elections on Sunday, including in Rome and Turin, in a major blow to Matteo Renzi, the Prime Minister.
In France, Marine Le Pen, the Front National leader, last night (Wednesday) called for France to have its own referendum on the "decaying" EU. "I would vote for Brexit, even if I think that France has a thousand more reasons to leave than the UK," she said.
In the Netherlands, polls show a majority of voters want a referendum on membership, and voters are evenly split over whether to stay or go.