Simon Coveney has accused Boris Johnson of putting the UK on a 'collision course' with the EU and Ireland in Brexit negotiations.
The Tánaiste added that the new British prime minister's first address to the House of Commons yesterday marked a "very bad day" in Brexit negotiations.
Mr Johnson again insisted that the UK will leave the bloc on October 31st, with or without a deal.
The Tory leader claimed to be eager to secure an orderly exit, but also suggested any deal must involve the "abolition" of the backstop to avoid a hard border in Ireland.
His remarks were swiftly dismissed by the Taoiseach and EU leaders - with chief Brexit negotiator Michel Barnier saying the backstop demand was "of course unacceptable".
The EU has repeatedly ruled out a renegotiation of the withdrawal agreement, which includes the backstop.
Minister Coveney, who met Mr Johnson's new Northern Ireland secretary earlier today, said the prime minister's comments were "very unhelpful".
He argued: "He seems to have made a deliberate decision to put Britain on a collision course with the European Union and Ireland in relation to the Brexit negotiations.
"I think only he can answer the question as to why he's doing that.
"I think it's been made very clear from the Taoiseach, Michel Barnier and Presidents [Donald] Tusk and [Jean-Claude] Juncker that the approach that the British prime minister seems to now be taking is not going to be the basis of an agreement - and that's worrying for everybody."
The Tánaiste said the Irish Government's contingency planning for a no-deal Brexit is already "intense", but it will remain a key focus for the Government to try and mitigate the damage a disorderly Brexit would cause.
He added: "From a Brexit negotiating perspective, it was a very bad day yesterday... we'll have to wait and see whether that message from London changes in the weeks ahead."