The global economy would be hit hard if President Donald Trump decides to impose steep tariffs on imported cars, Citi's Willem Buiter told CNBC on Thursday, with Germany likely to bear the brunt of any losses.

The U.S. administration recently declared that some imported vehicles and autos parts pose a national security threat to the world's largest economy.

It comes after the Commerce Department submitted its so-called "Section 232" national security report in February. Specific recommendations from this report have not been revealed, but it is widely believed to have concluded that car and auto part imports pose a risk on national security grounds.

However, Trump stopped short of imposing auto tariffs last week, choosing instead to delay the decision for as long as six months to allow for more time for trade talks with the European Union and Japan.

"If we get in a position of tariffs under the section 232 act that the U.S. is threatening with, for which it has given a reprieve until November, then that would be serious," Willem Buiter, special economic adviser at Citi, told CNBC's "Squawk Box Europe" on Thursday.

"That would mean tariffs for car imports, for car parts imports, and that would hit Germany the most of any of the large industrial nations. That would be a first-order slap in the economic face," he added.