London/Brussels - EU officials accused new British Prime Minister Boris Johnson of taking a "combative" approach after he demanded in his first speech to parliament that Brussels renegotiate Brexit or face the prospect of Britain leaving without a deal.
The European Union's chief Brexit negotiator Michel Barnier on Thursday urged the bloc's 27 remaining member states to "remain calm, stick to our principles and guidelines and show solidarity and unity."
The bloc will not heed Johnson's call to drop the Irish backstop clause in order to save the withdrawal deal, Barnier wrote in a letter to EU leaders seen by dpa.
Barnier said Johnson's statement was "rather combative," adding that he remains "available throughout the summer for talks" with Britain.
Johnson earlier Thursday urged EU leaders to "rethink their current refusal" to renegotiate Britain's EU withdrawal agreement.
"We are ready to negotiate, in good faith, an alternative [Brexit deal]," Johnson told British lawmakers. "We will throw ourselves into these negotiations with the greatest energy and determination."
If the EU does not renegotiate, "we will have to leave without an agreement," he said, adding that he has asked his new ministers to make preparations for a no-deal Brexit their "top priority".
The president of the European Commission, Jean-Claude Juncker, discussed the withdrawal agreement with Johnson by phone later Thursday, according to a spokeswoman.
"President Juncker listened to what ... Prime Minister Johnson had to say, reiterating the EU's position that the withdrawal agreement is the best and only agreement possible," she told dpa.
The commission said earlier that it "will not reopen the withdrawal agreement when it comes to the [Irish border] backstop question," referring to the key sticking point in previous negotiations.
The backstop is an insurance policy - in the absence of a better arrangement - to keep open the border between Northern Ireland, which would leave the EU with the rest of the United Kingdom, and the Republic of Ireland, which would remain a member of the bloc.
The EU has said it is willing to add to the political declaration on the future relationship, but not willing to change the withdrawal deal itself.
"We expect the United Kingdom to live up to its commitments to avoiding a hard border while protecting Ireland's place in the [EU] internal market," Juncker's spokeswoman Mina Andreeva said.
Johnson insisted that Britain must leave the EU by October 31, with "no ifs, no buts," with or without a deal.
"By 2050, it is more than possible that the United Kingdom will be the greatest and most prosperous economy in Europe - at the centre of a new network of trade deals that we have pioneered," he told parliament.
Johnson announced sweeping changes to his cabinet after he took office on Wednesday, appointing more eurosceptics to senior posts.
He has reportedly appointed controversial former Vote Leave campaign director Dominic Cummings, who was seen in photographs of Johnson at Downing Street on Wednesday, as an adviser to his new government.
Pro-EU lawmaker Nick Boles, who campaigned for a softer Brexit and left the Conservatives in April, tweeted that "the hard right has taken over the Conservative party."