The EU's chief negotiator for Brexit Michel Barnier | Tobias Schwarz/AFP via Getty Images Britain furious with Barnier over ISIS claim EU negotiator’s assertion is ‘irrational and stupid,’ UK official said.

LONDON — Michel Barnier has provoked fury in Whitehall by saying that, in voting for Brexit, Britons chose not to stand “shoulder to shoulder” with the EU just six months after a French call for solidarity against ISIS.

A U.K. official close to the Brexit negotiations said the claim was tantamount to “accusing us of ducking out of the fight against Daesh [ISIS].” The official, speaking on condition of anonymity, branded Barnier’s assertion “irrational and stupid.”

The row threatens to sour the atmosphere of Brexit negotiations, just two weeks ahead of December's critical European Council summit.

The European Commission’s Brexit negotiator was addressing a security conference in Berlin Wednesday morning.

Referring to the vote to leave the EU in June 2016, Barnier said: “It was a decision that came after a series of attacks on European soil, committed by young people who grew up in Europe, in our countries. It was a decision that came six months after the French minister of defense issued a call for solidarity to all his European counterparts to join forces to fight the terrorism of Daesh.

“Never had the need to be together, to protect ourselves together, to act together been so strong, so manifest. Yet rather than stay shoulder to shoulder with the Union, the British chose to be on their own again.”

Barnier was referring to calls from France's then defense minister, now foreign minister, Jean-Yves Le Drian, for European solidarity in the wake of the November 2015 terror attacks in Paris that left 130 dead.

The U.K. has played an active part in military operations against ISIS in Iraq and also, since a House of Commons vote in December 2015, Syria. The British official said the U.K. was the second largest contributor to the U.S.-led coalition’s campaign against ISIS, and also a “key player” in the humanitarian response to the conflicts in Syria and Iraq. The official also pointed out the U.K.’s role in training Iraqi state security forces.

Questioned about Barnier’s speech, Theresa May’s official spokesman emphasized the “leading role Britain has played and continues to play in combating Daesh.”

The prime minister has been absolutely clear about our commitment to the ongoing security of Europe and of the European Union and her determination to continue cooperating with the EU, sharing information and standing together against IS and terrorism in all its forms.”

Stefaan De Rynck, an advisor to Barnier, said on Twitter that the EU’s chief negotiator had merely “stressed partnership defence/security w/ UK: NATO, bilaterally and w/ EU (but EU autonomy), unconditional.” [sic]

In the same speech, Barnier made clear that the U.K. would have no decision-making role in EU foreign affairs and defense structures, but could continue to participate in joint operations.

The U.K. regards its military strength and intelligence and security expertise as among its strongest cards in the Brexit talks, which are approaching a critical phase ahead of December’s European Council meeting, where U.K. negotiators hope European leaders will allow talks on a transition period and a future trading partnership to begin.