Boris Johnson has sent his most senior EU adviser and Brexit negotiator to Brussels to deliver in person his message that the UK will leave without a deal unless the bloc abolishes the Irish backstop.

David Frost, a former British ambassador to Denmark who was also an adviser to Johnson when he was foreign secretary, is to hold talks with EU officials over the next 48 hours.

As Johnson’s choice to replace Olly Robbins, Frost is to be the new government’s main interlocutor for fresh negotiations. His contact is the most significant so far between Johnson’s administration and Brussels.

He will meet Clara Martinez Alberola, the head of cabinet for the European commission president, Jean-Claude Juncker; Stéphanie Riso, a senior official in Michel Barnier’s negotiations taskforce who was a key player in drafting the terms of the backstop, and Ilze Juhansone, the deputy secretary general at the commission.

The meetings will be held over Wednesday afternoon and Thursday morning but a government spokesman reiterated the prime minister’s position that without a radical EU rethink of the backstop there would be no significant talks.

Johnson has insisted that removal of the “undemocratic” backstop – which would keep Northern Ireland under single market regulations and the whole of the UK in a customs union to avoid a hard border on the island of Ireland – is necessary for a deal to be struck on the terms of the UK’s departure.

A government spokesman 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.”

During a phone call on Tuesday, Barnier told the Brexit secretary, Steve Barclay, that there was no chance of the EU changing its position or offering a “managed no deal” through side-deals to cushion the economic impact.

The EU’s chief negotiator “confirmed that the EU no-deal measures are unilateral in nature and aim at the protection of the EU27 interests”, a spokesman said of measures already announced to keep planes in the air and haulage routes open for up to nine months.

Frost’s appointment has been well received in Brussels, where he is a known and respected figure. He first worked in Brussels at the UK’s permanent representation to the EU in 1993 and went on to work closely with the current UK ambassador to the EU, Sir Tim Barrow, in the European Union department, of which he would later become director.

From May 2006 until October 2008, Frost was the ambassador to Denmark, after which he became the chief executive of the Scotch Whisky Association. He returned to the Foreign Office as Johnson’s special adviser between 2016 and the foreign secretary’s resignation in 2018.

Frost moved to his current role in Downing Street from his job as chief executive of the London Chamber of Commerce and Industry.

Diplomatic sources who know Frost have counselled against any suggestion that he would be sympathetic to remaining in the EU. He is understood to believe the UK should leave both the single market and customs union in order to benefit from Brexit.

In recent years he has called for better no-deal preparations by the government. Commenting on Theresa May’s ousting, Frost tweeted: “The prime minister’s departure is an unavoidable necessity for moving beyond the country’s political log-jam.”

Johnson has demanded the ditching of the backstop in phone calls in recent days with the Irish prime minister, Leo Varadkar, the German chancellor, Angela Merkel, and the French president, Emmanuel Macron.

Writing for the Guardian on Wednesday, the European parliament’s Brexit coordinator, Guy Verhofstadt, echoed the recent comments by all EU leaders by insisting the withdrawal agreement would not be reopened.

“Faced with a British government intent on ratcheting up talk of no deal, other European governments have no choice but to prepare for the worst too, but this is far from a desirable path,” he said.

“In the face of such irresponsible posturing, far from feeling threatened, I fully expect EU governments to remain calm and keep their unity. Attempts to put pressure on Ireland will only be met with waves of solidarity from the rest of the EU.”