The latest move is just part of worsening tensions between the new US administration under the 70-year-old Republican and EU leaders.
The warning was issued by Ted Malloch, who is Mr Trump’s choice for the job within the EU bloc.
Prof Malloch said on the BBC’s Pienaar’s Politics yesterday: “There would be an equal diplomatic measure in Washington.”
US President Donald Trump is in a war of words with the EU
He said: "Trump hopes that Europe succeeds completely and he wants to deal with them bilaterally.
"But obviously, he doesn’t want a union that is tilted towards Germany."
He added: "The hope is that the EU can come to some understanding and the union itself, which is in a process of adjustment, will see its role as something more of an economic integration, rather than political."
Nigel Farage (R) with Ted Malloch (L)
He continued: "Trump won’t cow down to the powers that be.
"He’ll speak his mind even if gets in trouble or held in disregard by others.
"It used to be called honesty but in the age of baby talk and political correctness, and mostly bulls**t, it's now regarded as dishonesty."
The European Parliament called on EU heads to block the British-based lecturer’s appointment, who is seen as a controversial figure, last week.
Being President Trump: Key Moments
Fri, February 3, 2017
Get Quotes on Home Insurance
Inside Donald Trump's first week in the White House.
1 of 14
Trump signs an order to review the Dodd-Frank Wall Street to roll back financial regulations of the Obama era.
Mr Talloch, 64, has previously claimed to want to “tame” the EU and has also predicted the collapse of the euro within two years.
He also ruffled feather last month by saying of the European Commission President Jean-Claude Juncker that he was a “very adequate mayor, I think, of some city in Luxembourg and maybe he should go back and do that again”.
Leading EU politicians are reported to have pleaded with top Brussels bosses Jean-Claude Juncker and Donald Tusk to refuse Professor Malloch official status as a US representative.
Influential members of the European Parliament have written to Mr Juncker and Mr Tusk to ask for EU institutions to treat the academic as a ‘persona non grata’ should Prof Malloch be appointed by President Trump.
In an interview with Bloomberg, Prof Malloch said: “They've never met me, talked to me, or read anything I've written so that would be a mischaracterisation.
“The letter, I think, is addressing the question of a future ambassadorship and whether a person should have only pro-EU sentiments or views.
European Commission President Jean-Claude Juncker with German Chancellor Angela Merkel
“Well, within the EU itself there are probably three or four different views on what the EU should be or where it may go.
“And in the Parliament the constellation is sprayed across the whole spectrum of opinion.
“But, frankly, the Americans get to pick who their ambassador to a country or a mission would be.
US President Donald Trump at his golf course
“It's never the case that a country gets to pick or a mission gets to pick who that ambassador is.”
The US and EU are currently engaged in an increasingly bitter war of words.
Guy Verhofstadt, the former Belgian Prime Minister and lead Brexit negotiator for the European Parliament, said last month: “I have just come back from US and my view is that we have a third front that is undermining the EU and that is Donald Trump.”