David Frost, a former Foreign Office official who has been appointed the so-called "sherpa" to the European Union, held meetings with officials, it said.

Johnson is demanding the EU drop its resistance to renegotiating key elements of the Brexit divorce deal struck last year with his predecessor Theresa May -- or face the prospect of Britain's disorderly departure from the bloc on October 31.

European leaders have insisted the withdrawal agreement, which sets out the terms of Britain's  departure after 46 years of membership, is closed.

But they are willing to reopen discussions on the future relationship contained in a "political declaration" accompanying the deal.

"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," a Downing Street spokeswoman said.

"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," she added, referring to a controversial measure in the withdrawal agreement aimed at keeping the Irish border open in all post-Brexit scenarios.

Frost, who left a prior role as chief executive of the London Chamber of Commerce and Industry, replaces May's top EU adviser Olly Robbins.

Robbins drew repeated criticism from ardent Brexiteers for being too sympathetic to the bloc in the Brexit negotiations.

Meanwhile Johnson on Wednesday met political leaders in Northern Ireland, which has become the focus of increasingly tense rhetoric on both sides of the Irish Sea over Brexit, hoping to build support for his new approach.