President-elect Donald Trump has predicted fresh departures from the EU, arguing that member-states will be keen to pursue their own identities.
However, European economic affairs commissioner Pierre Moscovici has told reporters that the "considerable" cost of Brexit would deter others.
Trump also said that Brexit would be " a great thing", and threatened German carmakers BMW, Daimler and Volkswagen with import tariffs as high as 35 per cent.
All three firms make cars in both the US and Mexico.
"If you want to build cars in the world, then I wish you all the best. You can build cars for the United States, but for every car that comes to the USA, you will pay 35 percent tax," Trump said in remarks translated into German.
"We must be extremely vigilant, mobilized and, when the times comes, reactive, if a certain spirit is confirmed," Moscovici responded.
"Europe must not be naive and Europe must be able to react."