David Frost, a former Foreign Office official, was sent over by the new Prime Minister to hold meetings with EU officials. The move comes as Mr Johnson has demanded the EU remove the Irish backstop from the Withdrawal Agreement. But the EU are currently resisting opening up key elements of the Brexit deal to be renegotiated.
A Downing Street spokeswoman said: “In his role as the Prime Minister’s Europe adviser, David Frost is visiting Brussels to have introductory meetings with key officials and to pass on the prime minister’s message in person.
“The UK is leaving the EU on 31 October whatever the circumstances. We will work energetically for a deal but the backstop must be abolished.
“If we are not able to reach an agreement then we will of course have to leave the EU without a deal.”
Mr Frost will be delivering the message that the UK will leave the EU on Halloween “whatever the circumstances”.
Spokeswoman Annika Breidthardt added: “I can confirm that David Frost is over the next days meeting with the head of the president’s cabinet Clara Martinez Alberola, with the secretary general ad interim Ilze Juhansone and Stephanie Riso, director in Task Force 50.”
She played down the prospect of Mr Frost’s meeting being the start of fresh negotiations.
“It’s a new sherpa, our officials always meet with the sherpa so it’s a meeting on that basis,” Ms Breidthardt said.
Mr Frost left his role as chief executive of the London Chamber of Commerce and Industry to take up his job in Mr Johnson’s administration.
He replaced Olly Robbins, whose efforts to negotiate a deal with Brussels had made him a target for Brexiteer criticism.
In an email to counterparts in the 27 other EU states, Mr Frost warned them not to “underestimate” Mr Johnson.
The missive, seen by the BBC, said “many people are inclined to underestimate Boris Johnson and I would urge you not to do so”.
He added “that you should be in no doubt about this Government’s commitment to the October 31 date”.