New prime minister Boris Johnson has promised to preserve the rights of EU27 citizens living in the UK after Brexit.
Speaking at his first parliamentary address as prime minister, the 55-year-old said he would guarantee with "absolute certainty" the rights of EU nationals in the UK.
But Labour leader Jeremy Corbyn said it was "a great shame" that the Conservative Party had only recently offered the guarantee after three years of "being treated as bargaining chips" that caused "untold stress and worry".
Johnson later said he wanted to avoid "a catastrophic loss of confidence" in politicians from the UK's population, reiterating his commitment to deliver Brexit by October 31, with or without a deal.
He promised in the meantime to work "flat out" to secure a deal with EU leaders, but said he didn't believe it would be possible without scrapping the contentious backstop arrangement.
"I do not accept the argument that all or part of the UK should remain in a customs union," he said, in reference to the border between Northern Ireland and the Republic of Ireland.
However, Corbyn said he was "deeply alarmed" by an apparent lack of planning for Brexit, and claimed Johnson was "flip-flopping" on his support of the backstop.
The new prime minister had months previously voted in favour of the backstop, Corbyn said.
EU leaders have repeatedly insisted that the deal negotiated by former prime minister Theresa May was the only one available, and renegotiation would therefore not be possible.
But Johnson said on Thursday he would "hope the EU will rethink their refusal" to make changes.
"If they don't," he said. "We will have to leave without a deal."
He added: "The UK is better prepared for the situation than many believe."
European Commission president Jean-Claude Juncker emphasised in a phone call with Johnson on Thursday afternoon the EU's position that "the Withdrawal Agreement is the best and only agreement possible", a readout of the phone call said.
Juncker said the Commission would be willing to hear new ideas from the UK provided they are "compatible" with the agreement. The two leaders exchanged "mobile phone numbers".
The EU's chief Brexit negotiator, Michel Barnier, called Johnson's talk of eliminating the backstop "unacceptable".
Barnier sent an email to EU member states, which warned that Johnson's "combative" speech was indicative of a no-deal outcome. He said it was essential that the EU "stick to our principles and guidelines and show solidarity and unity of the 27".
The former journalist then pledged to "begin right away" on further economic preparations, including tax cuts to boost innovation, accelerating talks on free trade deals, and devising ways to boost British businesses overseas.
He painted an idyllic picture of his "green, clean, prosperous, confident and ambitious" vision of the UK's post-Brexit future that would be "the greatest place on Earth".
"By 2050, it is more than possible that the UK will be the greatest and most prosperous economy in Europe," he added.
But Corbyn was less sure, saying the UK was "deeply concerned" that the new prime minister "overestimates himself".
Taking to the despatch box, Corbyn compared his rival to US President Donald Trump and criticised the "reckless race to the bottom" for Brexit.
"People do not trust this prime minister to make the right choices for the majority of people in this country," he said.
You can watch the address again by clicking on the player above.