BERLIN - The European Union will not be bullied into compromising its principles, Norbert Roettgen, a German lawmaker and ally of Chancellor Angela Merkel, said in a warning to British Prime Minister Boris Johnson.

On entering Downing Street on Wednesday, Johnson set up a showdown with the EU saying that he would negotiate a new Brexit deal and threatening that, if the EU refused, he would take Britain out on Oct. 31 without a deal on its future relations.

Roettgen, chairman of the German Bundestag’s Committee on Foreign Affairs, said in a tweet late on Thursday: “Dear @BorisJohnson: Neither boastful speeches nor bullying will succeed in making us give up #EU principles and unity.”

“Will stay cool instead,” Roettgen said. “Sadly, both in words and deeds - has appointed a cabinet of #Brexiteers - #Johnson fails to reach out to country and continent.”

European Commission President Jean-Claude Juncker told Johnson that a divorce deal agreed by his predecessor in November was the best and only deal on offer with the EU, an EU spokeswoman said on Thursday.