Boris Johnson has set the UK on an apparent course towards a no-deal Brexit by playing down the likelihood of any talks with the EU unless Brussels agrees to scrap the existing withdrawal agreement and Irish backstop, both of which it has ruled out.

The seemingly intransigent tone prompted Leo Varadkar, the Irish prime minister, to warn that a no-deal departure could lead more people in Northern Ireland to seek a united Ireland.

Johnson’s spokesman said there were no talks with the EU scheduled, and that if they happened it must be “clear what the basis for those discussions needs to be” – a message the new PM then delivered personally to Angela Merkel.

According to No 10, Johnson told the German chancellor in a phone call on Friday that while he would seek a deal, the “only solution” involved entirely scrapping the backstop, and that in the interim the UK must prepare for the prospect of there being no deal.

Asked earlier if Johnson’s team had sought any talks with Brussels, a Downing Street spokesman said: “What we’ve done is set out our position and say that we are very ready and will be energetic in beginning talking, but we’re also clear-eyed about what needs to happen if we are going to be able to secure a deal which parliament can support.

“As I say, we are ready to begin talking, but we are clear what the basis for those discussions needs to be.”

The approach from Johnson, who spent Friday visiting a police centre in Birmingham to tout his domestic promise of 20,000 new officers, will boost speculation about a deliberate attempt by a No 10 now packed with former Vote Leave alumni to scupper any negotiated settlement.

With a seeming majority in the Commons set on blocking a no-deal departure on 31 October, this could create an impasse in which Johnson could declare an autumn election, for which he will be styled as the champion of Brexit.

The strategy does, however, carry risks, not least to the integrity of the UK, even as No 10 confirmed that Johnson would carry the extra, self-appointed title of “minister for the union”.

On the visit to Birmingham, Johnson tried to scotch speculation that he was preparing for a general election, saying he would “absolutely not” call for another vote. The prime minister said: “The British people … don’t want another electoral event, they don’t want a referendum, they don’t want a general election. They want us to deliver.”

While Johnson has no plans for talks with Brussels, he has spoken to Donald Trump, who called him on Friday evening to congratulate him on his new role and discuss the “unparalleled” trade opportunities offered by Brexit. The US president, who has publicly praised Johnson on numerous occasions, also discussed the Iran crisis.

Downing Street said: “The leaders both expressed their commitment to delivering an ambitious free trade agreement and to starting negotiations as soon as possible after the UK leaves the EU.”

However, at an event on Friday in Co Donegal, Varadkar warned that a no-deal Brexit could make some in Northern Ireland “question the union”. “People who you might describe as moderate nationalists or moderate Catholics, who were more or less happy with the status quo, will look more towards a united Ireland,” he said.

“And we will increasingly see liberal Protestants and liberal unionists starting to ask the question as to where they feel more at home: is it in a nationalist Britain that is talking about potentially reintroducing the death penalty, or something like that, or is it part of a European home and part of Ireland?”

Johnson is likely to face some turbulent encounters with other EU leaders, including at next month’s G7 summit in France. Following Johnson’s call with Merkel, a German spokeswoman, Ulrike Demmer, said he had also accepted an invitation to visit Berlin.

He is expected to combine it with a visit to the French president, Emmanuel Macron, to whom he also also spoken. Political insiders in Berlin say the visit could take place as early as next week, but no date has been announced.

Michael Roth, Germany’s minister of state for Europe, said Johnson was mistaken “to believe that the European Union will let itself be blackmailed”. He told the TV channel ZDF: “My message to the new British prime minister is very clear. Boris, the election campaign is over. Just calm down.”

Following Johnson’s talk with Macron, France’s minister for European affairs, Amélie de Montchalin, said both sides should avoid “games, gestures and provocations” with just three months to go before the UK is due to leave the EU.

On the domestic front, Johnson is due to make the latest in a series of ambitious, if so far vague and uncosted, new policies, this time for a new trans-Pennine rail route between Manchester and Leeds.

At a speech in Manchester on Saturday, Johnson was expected to say: “It will be up to local people and us to come to an agreement on the exact proposal they want – but I have tasked officials to accelerate their work on these plans so that we are ready to do a deal in the autumn.”

The coming weeks, however, look set to be dominated by what could descend into a Brexit standoff, with a united EU27 adamant that there is no time to rewrite the withdrawal agreement, and that the backstop is the only proven way to avoid the risk of a hard Irish border.

Pressed on whether Johnson’s stance meant, in effect, that the process was deadlocked, his spokesman said the UK’s priority remained to leave with a deal. “But it is very clear that if we are going to be able to secure a deal that allows us to leave on 31 October, which the prime minister has said we are absolutely going to do, that is going to mean reopening the withdrawal agreement,” he said.

Johnson must also factor in concerns about his strategy from worried Conservatives, given that he has a working Commons majority of just three, which seems set to fall further after the Brecon and Radnorshire byelection next week.

Tobias Ellwood, the junior defence minister, who strongly opposes a no-deal departure, was sacked from his post on Friday. He later told the Press Association that while “pure Brexiteers” on the Conservative benches wanted no deal, “the damage to our economy will send our party into opposition for a decade”.