European Economic and Financial Affairs Commissioner Pierre Moscovici meets with Greek Prime Minister Alexis Tsipras (not pictured) at the Maximos Mansion in Athens, Greece, November 28, 2016. REUTERS/Michalis Karagiannis

PARIS (Reuters) - It’s fantasy to think other European countries will follow Britain in deciding to leave the European Union, a top European official said on Monday, after U.S. President-elect Donald Trump said he believed it would be the case.

Asked about Trump’s comments in an interview with the Times of London newspaper, European Economic Affairs Commissioner Pierre Moscovici said the cost of Brexit would be “considerable” and that it would deter other countries to follow suit.

“I’m not worried, I think this idea that Brexit is going to be contagious is a fantasy, a bad fantasy,” Moscovici told reporters in Paris.

“Brexit is not a great thing,” he said and warned Trump that comments advocating a break-up of the European Union would not get the trans-Atlantic relationship off to the best start.

Europe’s best response until Trump’s inauguration would be to remain in “wait-and-see” mode and watch the first steps of its new administration.

But asked about Trump’s comments on slapping tariffs on German carmakers such as BMW which sought to import cars to the U.S. from plants in Mexico, Moscovici said:

“We must be extremely vigilant, mobilised and, when the times comes, reactive, if a certain spirit is confirmed.”

“Europe must not be naive and Europe must be able to react,” he said.