Boris Johnson has clashed with Brussels over his call for the EU to drop its opposition to a new Brexit deal and return to the negotiating table.
In his first statement to MPs as British prime minister, Mr Johnson said he would work "flat out" to secure a new agreement on Britain's withdrawal from the EU.
But Brussels responded swiftly, with European Commission president Jean-Claude Juncker using his first phone call with the new prime minister to say the existing Withdrawal Agreement was "the best and only" deal possible.
Mr Johnson said the British government was "turbocharging" preparations for a no-deal break on October 31 if the EU refused to engage.
In Brussels, the EU's chief negotiator Michel Barnier said Mr Johnson's demands were "unacceptable" and accused him of using "combative" language to put pressure on the remaining EU27.
A European Commission spokesman said Mr 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".
He said he would be prepared to "analyse any ideas put forward by the United Kingdom, providing they are compatible with the Withdrawal Agreement".
The two politicians exchanged mobile phone numbers and agreed to remain in touch "should the United Kingdom wish to hold talks and clarify its position in more detail".
Mr Johnson appeared in the Commons chamber to cheers from Tory MPs, still reeling after his brutal purge of Theresa May's cabinet which saw 17 ministers sacked or quit their jobs.
He underlined his determination to take Britain out of the EU by the end of October, warning that failure to do so would lead to a "catastrophic loss of confidence" in the political system.
Despite the fears of many MPs he is setting Britain on course for a no-deal break, Mr Johnson insisted he would still prefer to leave with a new agreement in place.
However he said Mrs May's Withdrawal Agreement – rejected three times by MPs – was "unacceptable" and that the Northern Ireland backstop had to go.
"No country that values its independence, and, indeed, its self respect, could agree to a treaty which signed away our economic independence and self government as this backstop does," he said.
The comments from Brussels echoed Taoiseach Leo Varadkar who said Mr Johnson's claims he could get a new deal by October 31 were "not in the real world".
Mr Johnson, who earlier chaired the first meeting of his new cabinet, insisted the UK side was ready to meet and talk with the EU, "whenever they are ready to do so".
"I would prefer us to leave the EU with a deal. I would much prefer it. I believe that is still possible even at this late stage and I will work flat out to make it happen," he said.
"For our part, we will throw ourselves into these negotiations with the greatest energy and determination and in the spirit of friendship.
"And I hope that the EU will be equally ready and that they will rethink their current refusal to make any changes to the Withdrawal Agreement."
At the same time he said he had ordered Michael Gove, the new chancellor of the Duchy of Lancaster in charge of no-deal preparations in the cabinet office, to "turbocharge" efforts to get the country ready for any eventuality.
As well as a major public relations campaign, Mr Johnson said there would be an "economic package" to boost business including changes to tax rules to incentivise investment in capital and research.
And he further sought to raise the stakes with the EU, saying he would not nominate a new UK commissioner - although he insisted that this was not intended to prevent the formation of the new commission due at the start of December.
However, Labour leader Jeremy Corbyn warned Mr Johnson was overestimating the ability of his "hastily thrown together... hard-right cabinet" to deliver a new Brexit deal.
"No-one underestimates this country but the country is deeply worried that the new prime minister overestimates himself," he said.
"People do not trust this prime minister to make the right choices for the majority of the people in this country when he's also promising tax giveaways to the richest of big business - his own party's funders."
However Mr Johnson drew cheers from Tory MPs with an attack on the Labour leader, accusing this "long-standing Euro-sceptic" of "metamorphosing" into a Remainer.
He also brushed off a call by the SNP's Westminster leader Ian Blackford for an immediate general election.
"The people of this country have voted in 2015, 2016, 2017 –- what they want to see is this parliament delivering on the mandate they gave us, including him," he said.
