Video report by ITV News Political Editor Robert Peston
Prime Minister Boris Johnson is on a collision course with Europe over Brexit with two of the most senior figures insisting the bloc will not change its position.
Britain's new leader told MPs in his first address he and his ministers are committed to leaving the EU on October 31 "whatever the circumstances", but they will not wait until Autumn to get to work.
Referring to Brexit, the prime minister said he would "prefer us to leave the EU with a deal", but said the Irish backstop must be abolished and the terms of the current EU withdrawal agreement were "unacceptable to this Parliament and to this country".
But, within hours, any hope of renegotiating the position appeared to be scuppered by both EU Commission chief Jean-Claude Juncker and lead negotiator Michel Barnier.
In a phone call, Mr Juncker insisted the existing deal was "the best and only agreement possible".
And Mr Barnier said the stance adopted by Mr Johnson was "unacceptable" to the 27 EU nations.
They were responding to Mr Johnson's maiden speech as leader in which he promised EU citizens "absolute certainty" they will be able to live and work in the UK after Brexit.
Speaking in the House of Commons, he said: "I, and all ministers, are committed to leaving on this date. Whatever the circumstances.
"To do otherwise would cause a catastrophic loss of confidence in our political system."
ITV News Europe Editor James Mates says the PM's demand may harden the resolve of the EU27
He said Michael Gove - who he has appointed Chancellor of the Duchy of Lancaster in the newly formed Cabinet - would make no-deal Brexit plans a "top priority" but admitted "we are not as ready yet as we should be."
Mr Johnson remained hopeful the EU would rethink their current refusal to make any changes to the Withdrawal Agreement, but kept by his pledge to leave without a deal under Article 50.
He vowed to "prove the doubters wrong" at this "pivotal moment".
However, the EU's top negotiator, Michel Barnier, said in a note to member states that Mr Johnson's call for changes by way of eliminating the backstop was "unacceptable".
He wrote: "But as suggested by his rather combative speech, we have to be ready for a situation where he gives priority to the planning for 'no deal', partly to heap pressure on the unity of the EU27."
And a spokeswoman for Mr Juncker said he told the PM: "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 - in line with the European Council guidelines."
Mr Johnson used his first Commons address to promise to "re-energise" the UK to "make it the greatest place on earth" by 2050.
Mr Johnson said citizens are likely to look back in 2050 and see today as the start of a "new golden age" for the UK.
The new prime minister pledged to get 20,000 extra police, hand them greater stop-and-search powers and to introduce an Australian points-based immigration system.
But in response to Mr Johnson's maiden speech in the Commons, Labour leader Jeremy Corbyn said he had thrown together a "hard-right cabinet", with the appointment of Vote Leavers.
Mr Corbyn asked if the Conservative party had any plans to bring back capital punishment, after newly appointed Home Secretary Priti Patel previously advocated its return.
The Labour leader said: "No-one underestimates this country but the country is deeply worried that the new Prime Minister overestimates himself.
"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."
Mr Corbyn asked the prime minister "to take this opportunity to rule out once and for all that our NHS is not going to be part of any trade deal with President Trump and the USA?"
In response, Mr Johnson said "under no circumstances would we agree to any free-trade deal that put the NHS on the table."
SNP Westminster leader Ian Blackford welcomed Mr Johnson to his post as the "last Prime Minister of the United Kingdom".
"It's often said the Prime Minister lives in a parallel universe - that's been proven beyond any reasonable doubt this morning, in fact it looked as if he was about to launch himself into outer space," he added.
Mr Blackford questioned the Tory party for their choice of prime minister, "what have they done?" he asked.
Earlier, Mr Johnson presided over his first meeting of his new Cabinet after a brutal cull of Theresa May’s top team.
Within hours of taking office on Wednesday, the new Prime Minister moved to stamp his authority, putting Brexiteers into key Cabinet posts as he vowed to take Britain out of the EU by the October 31 deadline.
In the most eye-catching appointment of a dramatic day, arch-Brexiteer Jacob Rees-Mogg – who proved such a thorn in the side of Mrs May – entered the Government as Leader of the Commons.
Unusually, however, Downing Street said that he would not be a full Cabinet member, although he will attend Cabinet meetings.
The most high profile casualty of Mr Johnson’s cull was his defeated leadership rival Jeremy Hunt, after he refused to accept a demotion from the Foreign Office.
Others to be sacked included Penny Mordaunt and Liam Fox, both prominent backers of Mr Hunt, who lost their jobs despite their record as committed Brexiteers.
In contrast, Dominic Raab, who quit the Cabinet over Mrs May’s Brexit deal, returned as Foreign Secretary, Priti Patel was made Home Secretary and Michael Gove became Chancellor of the Duchy of Lancaster in the Cabinet Office.
Stephen Barclay, another prominent Brexiteer, retains the key post of Brexit Secretary.
In another potentially controversial move, Mr Johnson brought in the abrasive Vote Leave mastermind Dominic Cummings in an advisory role.
Meanwhile, former Prime Minister Theresa May is enjoying a day off, after resigning from the top job in the country.
She is watching day two of the Test Series at Lord's on her first day as a backbench MP, alongside Gavin Barwell, David Gauke and Greg Clark.
A large yellow removal van was seen outside Downing Street on Thursday morning, from a company called Bishop's Move.
It was reversed up the street before coming to a stop outside Number 11.
Following his address to the Commons, Mr Johnson is expected to continue with middle-ranking and junior ministerial appointments.
But with more than half of Mrs May’s Cabinet having quit or been sacked, he is well aware that he could face a difficult time at Westminster.
It prompted speculation that he could hold a snap general election after MPs return in September – once they have finished their summer break which starts on Friday – to try to break the deadlock.